Pit Stop with Marlan Davis: How Hard is a Hydroboost Brake System Conversion on a Chevy 350-Powered 1940 Ford Fat-Fender Coupe with Low Idle Vacuum That Won’t Stop? Are There Any Other Solutions?
Many 19992007 GM full-size trucks and SUVs came with Hydroboost power brakes fed by the power steering pump in place of vacuum-assisted brakes linked to engine vacuum. Kinderman wants to use Hydroboost brake system on his 1940 Ford street rod.
Photo: Jesse Kiser
I have question about Hydroboost brakes. The cam in my 1940 Ford Coupe doesn’t make a lot of vacuum. I would like to put power-steering and Hydroboost brakes on the car. My son has an early 2000 Chevy Tahoe with Hydroboost brakes. There should be a lot of those in junkyards. Could I pull off the power steering pump, Hydroboost unit, and hoses and install them on my car? I know the hoses won’t fit but there is a place in town that can make just about any hydraulic hose.
Rich Kinderman’s 1940 Ford fat-fender street rod looks fast just standing still.
Photo: Rich Kinderman
Power comes from a Chevy 350 small-block with a healthy Comp Cams Xtreme Energy flat-tappet hydraulic cam. With 230/236-degrees duration at 0.050-inch tappet-lift, there’s not enough low-end vacuum for traditional vacuum-assisted power brakes.
Photo: Rich Kinderman
|KINDERMAN’S 1940 FORD COUPE: ADDITIONAL INFORMATION|
|Direct conversation with Mr. Holmes provided this additional information.|
|Engine||Chevy 355 legacy small-block (350 +0.030″)|
|Intake manifold||Edelbrock RPM Air-Gap high-rise dual-plane|
|Carburetor||Quick Fuel 650 cfm|
|Camshaft||Comp Cams Xtreme Energy hydraulic flat-tappet XE274H, 230/236 duration @ 0.050″ tappet-lift, 0.490″/0.490″ lift (1.5:1 rockers), 110 LSA|
|Front suspension||Stock Mustang II front-end “cut out of a Mustang and welded to 1940 chassis”, new coilovers and tubular control arms|
|Steering||Mustang II manual rack-and-pinion (will install power rack)|
|Front drive system||V-belts (“I’m old-school and don’t want to go serpentine. “)|
|Brakes, front||Wilwood Dynalite 4-piston calipers on 11″ rotors (“clears my Torque Thrust D 14″ wheels”)|
|Brakes, rear||1979 Ford F-150 9″ rearend with 11×2 drums|
|Brake master cylinder||Late 1970s Corvette dual master, 1-1/8 inch piston-bores|
|Brake power assist||Vacuum: “Small 7″ to 8″ od x 4″ to 6″ OL booster”|
|Brake pedal||Floor-mount, linkage connects to underfloor master-cylinder and vacuum-booster|
|Problem||“Brakes only work well on top-end; pedal is firm at low speeds, but car won’t stop. “|
|Idle vacuum||8 to 10 in-Hg|
For reliable vacuum-assisted brake operation, engine idle vacuum should be at least 14 inches (and preferably 16 inches or more). With only 810 inches at idle from Kinderman’s 350, there’s no way even a huge vacuum-booster could pull out all the stops—but Kinderman only has room underfloor for this tiny 8-inch booster. He says the big master-cylinder is out of a 1979 Corvette.
Photo: Rich Kinderman
Wilwood four-piston Dynalite calipers on 11-inch o.d. rotors should be adequate for the job, assuming sufficient brake pressure. They’re paired with Ford 11 x 2-inch drums out back. Kinderman cut out a rack-and-pinion Mustang II front-end from a real Mustang and grafted onto the 1940, then later enhanced with tubular control arms and coilovers.
Photo: Rich Kinderman
Rich, we see problems like yours quite often on a variety of cars, and there are several solutions, including your proposed “Hydroboost” brake system. Technically known as a hydraulically boosted brake system, instead of vacuum, a Hydroboost uses hydraulic fluid supplied by the same power-steering pump that supplies fluid to the steering-gear or steering-rack to boost brake pedal force. Usually an accumulator supplies some short-term reserve boost in case of an engine stall or pump failure.
First, Check Your Engine
Before you start swapping parts, I strongly recommend double-checking your current vacuum-boosted brake system setup is as good as it can get. The Comp XE274H cam is pretty healthy for a 350 engine, but your reported vacuum output still seems on the low side. You might gain 2 to 3 inches of vacuum with a professional dyno-tune. On one of Comp Cam’s 350 test engines an XE274H-10 hydraulic flat-tappet (PN: 12-246-3) had 11 inches at 800 rpm and 14 inches at 1,000 rpm under no load. Interestingly, a hydraulic-roller XR282HR-10 (PN 12-432-8) with the same 0.050 duration but more lift had 9 inches at 800 and 11.5 inches at 1,000 rpm. “The [roller cam’s] slightly softer ramps at low lift and more area under the curve further up both work to make the heads and cam ‘act’ bigger,” reports Comp’s Billy Godbold. On the other hand, “the roller was worth about 30 more horsepower in this engine, too.”
Have you tried advancing the initial timing to see if idle vacuum improves? If it does, but now there’s too much timing on the top end, you’ll need to go through the distributor and play with the centrifugal advance-curve.
Adding more base timing can improve engine vacuum and idle quality, but if the engine likes a lot of initial lead, you might have to back down the advance curve to prevent top-end detonation.
Photo: Marlan Davis
A mistuned or bad carburetor can contribute to low vacuum output. This could be as simple as proper idle mixture adjustment or checking whether you have the right power valve installed. If the “PV” opens at idle, it will dump raw fuel into the manifold plenum. That’ll mess up your vacuum and driveability for sure! A crude fix for that is a 3.5-inch power valve with richer main jets, but that might kill cruise mileage. For a granular level approach, ace tune master Norm Brandes (Westech Automotive of Wisconsin) is practically within spitting distance of your location. The full-service shop takes a wholistic or “systems” approach to problems like yours. If—and it’s a big “if,” based on Comp Cam’s reported vacuum readings with your cam—a good tune can get that idle vacuum up to 14 inches, you might be able to scrape by with an external vacuum cannister (vacuum reserve tank).
Adjust the carburetor idle mixture screws to obtain the highest possible vacuum reading. Turn the curb idle speed screw slightly clockwise only as necessary to prevent the car from stalling out while doing this. A 15 in-Hg gauge reading here isn’t too bad for a hot rod, but be sure the power-brake vacuum booster can get the job done at that level. (Hint: Don’t rely on a minibooster. )
Photo: Marlan Davis
Second, Check and Refine Your Existing Brake System
I know this seems trite, but: Are you absolutely sure the brake system is properly bled? Sometimes it’s hard to get all the air out of the system, especially with a floor- or frame-mount master-cylinder location. Ideally, the master-cylinder reservoir should be the highest point in a brake system, so brake fluid flows downhill (thanks, gravity!).
Another thing we see on many hot rods is the vacuum booster is too small and/or poorly matched to a master cylinder that’s too large for the tiny booster. Yes, 1970s Corvettes had 1-1/8 inch bore master-cylinders (as did many GM power brake systems of that era), but it also had a large dual-diaphragm long-length vacuum booster. There’s no room for that type of booster underfloor, but Scarebird brake expert Mark Janis says there are many 9-inch o.d. dual-diaphragm vacuum boosters used on GM cars in the 1980s that would offer a noticeable improvement over what you have now. Many of these installations had a square mounting-bolt pattern/perpendicular mounting bracket that makes them well-suited for custom installations. One common unit are code “BK” boosters found on many 19821987 Chevy S10/GM S15 pickups, Jimmys, and Blazers.
You’ve told me the brakes work fine at the end of the dragstrip, just not at low speed. Sometimes that’s because of overly hard brake pads that don’t really grab well until they get hot. Great for racing, not necessarily for daily street use. Consider “softer” pads that work better at moderate temperatures typical on the street. Brake pads are usually marked with code letters on their backing pads or pad edges that provide a guide to the pad’s characteristics; for more information, see “Disc Brake Pad Friction Codes Explained.” If you do change to a different pad material, be sure to turn the rotors to remove any glaze and/or pad friction-material compound that may have transferred to the face. Otherwise, it’s like an engine cylinder-bore where the piston rings never seated.
For a master-cylinder, consider dropping down to a 1-inch unit. Make sure your pedal-ratio is correct and that you are achieving the full stroke of your master-cylinder without going over-center. Generically, power brake systems want about a 4:1 pedal ratio and manual brakes like 6:1, but about one “ratio number” higher usually isn’t a deal breaker.
There’s also the option of reverting to full-manual brakes. “A small power booster is worse than properly setup full manual brakes,” points out Janis. “I’m guessing about a 7/8-inch bore manual master would work pretty well for you. But watch that pedal ratio!”
Hydroboost Brakes Have Good Stopping Power
Now let’s go through what it might take to implement your plan to use early 2000s truck-based GM hydraulically-boosted brake systems. In general terms, Hydroboost brake systems provide excellent stopping ability, though some report the brakes as overly sensitive or positive; in other words, the pedal may lack linear modularity, which is important to a canyon carver or road racer, not so much for everyday cruising. On the other hand, many like the “extremely positive” pedal application sensitivity provided by a dialed-in hydraulically boosted brake system. Like most hot rod mods, in a non-stock installation you must be prepared to fine-tune and overcome unexpected issues.
Don’t Forget the Steering Rack
To start, you’re going to need a power-steering rack-and-pinion unit to replace your manual rack assembly. A 19741978 Mustang II power rack doesn’t grow on trees—if there are any trees in the wrecking yard. At RockAuto, a remanufactured Mustang II power rack is in the $120 range, while brand new ones go for $235 and up. There are both two- and three-mounting bolt designs, but the manufacturers say they interchange.
GM Truck Hydroboost Donor Candidates
Virtually the same Hydroboost brake system was used on many 19992007 small-block Generation III (LS-type) Chevy and GMC – -, and 1-ton pickups, Tahoes, Yukons, Sliverados, and related relatives and clones. While these systems are mostly similar, the safe route is getting a matched system from the same vehicle. Take everything, including the master cylinder, the Hydroboost unit, the hoses, the power steering pump, and the power steering cooler. Even if they won’t be the right length for your nonstock application, the original hoses could be important because the hose ends on the high-pressure side can serve as a template or even—with their typical specially-bent metal section behind the fitting—be reused by your local custom hose shop when they build you a custom hose.
Hydroboost Brake System Layout
This is the layout of a basic hydraulically boosted brake system. Most OE set-ups use a full-size Saginaw Type P pump with a unique three-hose reservoir. A single high-pressure line (Hose 1) exits the pump and routes to the Hydroboost behind the master-cylinder. High-pressure Hose 2 runs from the Hydroboost brakes to the steering gear or rack-and-pinion. Two separate low-pressure return lines—one from the Hydroboost and one from the steering gear or rack—return fluid to the pump. If you can’t find a three-hose reservoir Tee the two return hoses together (detail, photo A). If you need to reduce pressure to just the rack, use an external Heidts valve (see photo B).
Photo: Ryan Lugo
A] A “standard” two hose power steering pump reservoir requires joining the two separate return hoses together with a Tee fitting. To prevent backflow to the brake hydraulic booster unit, connect the hoses to the Tee as shown here.
Photo: Marlan Davis
B] One way to reduce pressure to the Mustang II rack while maintaining full pressure to the Hydroboost is with Heidts adjustable valve (PS-101, basic unit; PS-102, valve with pressure gauge). Connect the high-pressure ports (bottom holes) in-line between the hydraulic brake booster and the steering gear or rack; the top holes are for the low-pressure return to reservoir. Turning the valve clockwise increases pressure (makes the steering more sensitive). Valve flow is nondirectional (in and out holes can be flipped).
Typical GM Hydroboost systems use a single high-pressure outlet hose that runs from the pump to the Hydroboost unit, another high-pressure hose from the Hydroboost to the power steering gear or rack, either hoses or tubing from the gear or rack to a power-steering cooler, and then finally back to the power steering pump. The cooler might range from just a length of tubing in front of the radiator to an actual cooler assembly with integral inlet and outlet tubing and hoses. There’s also a separate return line from the Hydroboost unit directly to the pump reservoir. In other words, there are usually two low-pressure return lines back to the reservoir and one high-pressure outlet hose from the reservoir for a total of three attachments, where “normal” power steering reservoirs have only two—one return and one outlet.
You’ll find that most of the trucks’ Hydroboost three-hose power-steering pumps have a power-steering cooler integrally attached to the power-steering gear return line (not the Hydroboost return line) that bolts to the front of the radiator core support. If you drive hard, it’s cheap insurance for the gearbox—snap it up with the rest of the power steering system.
Photo: Jesse Kiser
Hydroboost Power Steering Pump
Because the power-steering pump itself is still the large Type C Saginaw pump with a 3/4-inch shaft diameter that’s been around since the late 1970s, it does accept legacy press-on V-belt pulleys. It does not accept mid-1970s and earlier bolt-on pulleys that fit a 5/8-inch shaft and attach via a keyway and locknut. Obviously, the LS brackets designed to bolt to the new engine and run a fixed serpentine belt system won’t work on an old small-block. Any V-belt drive pulley must have the correct offset for compatibility with the other pulleys, brackets, and water pump length. With V-belt drives, use a standard-rotation water pump (clockwise rotation, if standing at the front looking towards the rear). If using pulleys of unknown origin rather than a matched drive system, be careful that the accessory pulleys don’t try to drive any multi-groove crank and water pump pulleys at different speeds. After all, a squeaky pulley gets the shaft!
GM used this Saginaw pump with three-hose reservoir on many truck and SUV engines, including 19971998 legacy small-blocks and 19992007 LS engines (2000 Silverado, shown). All these applications had serpentine belt drives, with the brackets differing between small-blocks and LS engines. Earlier press-on V-belt pulleys for a 3/4-inch pump-shaft fit.
Photo: Jesse Kiser
Depending on your old small-block’s drive-system, you might need a different reservoir to match up properly with legacy brackets. GM dealer parts books (thanks, John Elway Chevrolet) show the 19992007 GM Hydroboost system truck power steering reservoir was used as far back as 1996, which would have been a legacy small-block, but still with a serpentine belt drive. Reservoirs from any Type P pump physically interchange, and some 1-ton small-block trucks reportedly came with Hydroboosts as far back as the early 1970s (but the early Hydroboost units themselves were heavy, cumbersome, and leak-prone). Be aware that any reservoir bracket mounting studs are metric on late pumps. Lee Power Steering is one source for correct integrated reservoirs, remote reservoirs, and V-belt pulleys.
Hydroboost Brake Conversion with Two-Hose Pump Reservoirs
If you must go with a vastly more available two-line reservoir, be careful how you tee the two return hoses together before they go into the single return-line port. Don’t connect both return hoses “on the run” so they directly face across from each other as this may cause backflow to the Hydroboost unit. Nevertheless, some sources, including Diversified Creations, claim that teeing the return hoses together sometimes causes a restriction and noise. Noise aside, a restriction creates heat that can shorten part life. For sure do not undersize the hoses. At least install hose and fittings with the same i.d. as those they are replacing. Don’t use standard rubber-core stainless steel braided “racing hose” on the pressure side; instead, use high-pressure Teflon-core hydraulic hose if you need to get fancy.
Steering Rack vs. Hydroboost Pressures And What Can Be Done About It
Maintaining the proper operating pressures are where you can run into trouble. The GM Hydroboost system design operating pressure is around 1,300 to 1,600 psi. That’s fine with the frame-mounted, recirculating-ball, power steering gearbox still used on those pickup trucks. But a Mustang II power rack is in the 900 to 1,200 psi range. Excessively high pressure delivered to the rack causes oversensitive (some say, “frightenedly twitchy”) steering. On the other hand, lowering pressure to the Hydroboost means, bluntly, less braking effectiveness. It’s a fine balancing act.
There are two ways to change the system pressure. Overall power steering pump pressure and volume is normally regulated by the flow-control valve assembly found behind the reservoir’s pressure-out fitting. The officially recommended method is to disassemble and tune this flow-control valve assembly. (Most power steering pump experts don’t recommend trimming the spring to change pressure.) The flow control valve can be carefully disassembled, and shims added or subtracted to change the pressure, as long as the spring inboard of the valve doesn’t go into coil-bind. Different orifice-sized flow-control valves are also available (bigger orifice, more volume; think of carburetor jets). This is hydraulic engineering at its finest; don’t be afraid to send the unit in for fine-tuning by a specialist like Lee Power Steering. The other way is a Heidts in-line adjustable valve that lets you keep full pressure to the Hydroboost while reducing pressure to the rack-and-pinion unit only.
To increase overall Saginaw power steering pump volume, drill out the orifice behind the reservoir’s “pressure out” fitting (A, design may vary). Behind this fitting is the flow control valve assembly (B). Increase pressure by disassembling valve and removing shims (C).
Photo: Marlan Davis
The best advice we can give you here is to take a cue from the full service and hot experts at Rollings Automotive: “We just hook up the wrecking yard part and see how it works. It may take a week or more to thoroughly get all the air out of the system before you can make a final determination to go ahead with more fine-tuning. Just don’t jump heavy into modding unless you don’t like the feel of the steering or brakes.”
Of course, there may be various mounting issues adapting your existing brake pedal and linkage to a Hydroboost unit and its complimentary master cylinder originally designed for modern firewall mounting with hanging brake pedals. I gotta ask, with all these variables, how much is your time and effort worth?
Aftermarket Hydroboost Brake Systems
Sweeting Performance specializes in custom-tailored Hydroboost brake kits for nearly any application. In most cases, it can supply units tailored to work with parts you already have like the master-cylinder and power steering pump or sell you everything needed for a one-stop conversion. Steel-braided hoses are a popular upgrade.
Photo: Sweeting Performance
Why reinvent the wheel? You can buy an aftermarket hydraulic-assist brake system completely tuned-in for what you’re trying to do—including underfloor/frame mounting—from niche outfits like Sweeting Performance or Hydratech that specialize in Hydroboost’s carefully setup for custom applications like yours. Although a completely matched pump, Hydroboost unit, and master-cylinder is ideal, often these experts can tailor a Hydroboost to work with your existing master cylinder and pedal linkage. Other options include modding the Hydroboost for compatibility with a lower pressure steering rack. “We can set up the Hydroboost to work with what you had before,” says Sweeting Performance owner Matt Sweeting. “We can mount it like you had before, we have all kinds of mounts and brackets. Nine out of 10 times, we can set it up for your master cylinder, whatever it is. No invention required.” Obviously, this requires a phone call, rather than just clicking off standard menu checkboxes.
Sweeting Performance’s hydraulic-assist units are optionally available powder coated in custom colors or standard black, an upgrade over the standard baked-on black paint, which, as owner Matt Sweeting puts it, “Is not just rattle-can black spray. It’s a good durable paint stock, but you will still see the casting bumps. Powder-coat will fill in the bumps and give you that rich, smooth look. “
Photo: Sweeting Performance
Sweeting offers a nearly endless array of applications. This one fits 19581962 Corvettes. It adapts to the original master cylinder pattern and is clocked so that it clears the hood and other obstacles. Note how Sweeting rotates this assembly to position the actuator (light gray cylinder) underneath for even more clearance.
Photo: Sweeting Performance
Sweeting offers various thickness spacers to set the Hydroboost unit back in cases where the distance from the firewall to the brake-pedal pin location is too short.
Photo: Sweeting Performance
The Electric Vacuum Pump Alternative
Finally, have you considered an electric vacuum pump? These pumps can mount anywhere—even in the trunk—because the only connection is a vacuum hose to the existing brake vacuum diaphragm, plus electrical wiring. Wiring is a heck of a lot easier to hide or loom out of the way then a Hydroboost, multiple hoses, and tubing. A vacuum pump simplifies your pedal hookup (other than you still need the proper ratio). You don’t obsess over steering rack versus Hydroboost pressure differences. Let us know how your brake system turns out and send us some pics of for final system layout, whatever it is.
One practical alternative to hydraulic brakes: Keep your small vacuum booster and provide fulltime vacuum with an electric vacuum pump. Summit Racing sells the well-reviewed Leed Brakes Bandit electric vacuum pump kits (shown) as well as its own “House” brand. Just be sure to minimize annoying vibration by insulating the pump mount with rubber doughnuts, and using adequate size wiring and a relay.
Photo: Summit Racing
Hydroboost Brakes: Thumbs Up or All Thumbs?
- Both thumbs Up: Good power brake solution for big-cam, low-vacuum engines.
- Missing Thumbs: Won’t help a mistuned or ailing engine that produces low vacuum.
- All Thumbs: Won’t crutch other brake system problems (wrong master cylinder, wrong pedal ratio or stroke, brake pads and rotor or drum brake shoes in poor condition or not properly sized, system not properly bled)
- Thumbs Up: Extremely positive brake pedal response when properly dialed in.
- Thumbing a Ride: System is so positive that pedal may lack linear “feel,” important to many roadracers and canyon-carvers.
- Broken Thumb: Older Hydroboost “salvage-yard” units can be big, bulky, ugly, heavy, and leak-prone.
- Thumb Pedicure: Newest slimmed-down OE systems as well as custom aftermarket Hydroboost kits are lighter, smaller, prettier, and extremely reliable.
- Thumbs Up: Modern aftermarket custom Hydroboosts have more clearance than giant-size dual vacuum-diaphragms.
- Thumbhole: Some of the new units even fit in nearly the same space as that mini-vacuum booster that can’t get the job done.
- Thumb Surgery: For the salvage-yard scrounger, Hydroboosts are all over the place but modding may still be necessary in a nonstock installation (at least try to get everything from the same vehicle to start).
- Thumbs Up: Custom, per-application, aftermarket Hydroboost kits take the guesswork out of Hydroboost conversions and minimize trial-and-error system fine-tuning.
- Thumb-nailed: On retrofits, the operating pressures of power rack-and-pinion steering and the Hydroboost may not be compatible without careful system tuning and additional parts.
- Thumbs plumber: Lots of hydraulic hoses and lines that can clutter up an engine bay.
- Let your thumbs do the walking: An electric vacuum pump is a practical alternative, as is (for some) just going back to manual brakes.
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Diversified Creations; Brighton, MI; 810.227.4777; DiversifiedCreations.com
Heidts Hot Rod and Muscle Cars; Lake Zurich, IL; 800.841.8188; Heidts.com
Hydratech Braking Systems LLC; Lebanon, TN; 615.449.8261; HydratechBraking.com
John Elway Chevrolet on South Broadway; Englewood, CO; 866.273.4757 or 303.761.5161; JohnElwayChevrolet.com/Order-Parts-Online
Lee Power Steering; Santa Clarita, CA; 661.568.9170; LeePowerSteering.com
Leed Brakes; Cheektowaga, NY; 716.852.2139; LeedBrakes.com
RockAuto LLC; Madison, WI; RockAuto.com
Rollings Automotive Inc.; Mira Loma, CA; 951.361.3001; RollingsAutoInc.com
Scarebird Classic Brakes LLC; Albuquerque, NM; Scarebird.com
Summit Racing Equipment; Akron, OH; 800.230.3030 (U.S.) or 330.630.3030 (outside U.S. ); SummitRacing.com
Sweeting Performance Engineering; 714.330.5286; Huntington Beach, CA; PowerBrakeService.net
Westech Automotive; Silver Lake, WI; 262.889.4346; WestechAuto.com
Wilwood Engineering Inc.; Camarillo, CA; 805.388.1188; Wilwood.com
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