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Hydro-Boost Brake Booster System for your Early Bronco

I have to admit that I have a thing for brakes… good ones, that is. Brakes and suspension were two areas I knew I wanted to improve dramatically when I first began my Bronco buildup eight years ago.

I began my braking endeavors by converting my aging 4 wheel drum setup to 4 wheel discs(unboosted) when I put a new set of axles under my truck in late 1993. I was a bit underwhelmed with the performance of the new brakes so I set out in typical engineer’s fashion of trying to read anything and everything I could get my hands on to learn about braking systems.

With a few changes and upgrades, I was able to achieve good braking performance, albeit with higher pedal pressures than most people like. After a few more tweaks in the intervening years, I added an aftermarket vacuum booster setup to the equation in late 1999. The booster meant, in general, less pedal pressure was required to generate the same braking force at the calipers. But in my personal experience, braking performance proved to be erratic. At times the brakes felt like they could have stopped a freight train. In other instances, pedal pressure exceeding that used with the manual setup could barely bring the truck to a stop. In one memorable case, I hit a pine tree with my front bumper because the booster seemed to be ineffective!

Booster testing and other diagnostics turned up no obvious deficiencies so, as with many other things on our trucks, I learned to live with the shortcomings until I could find a suitable replacement. Shortly after installing the vacuum booster, I started looking around for an alternative that would give good braking without the high pedal effort of the unboosted setup. I immediately started thinking about hydroboost. I first saw a hydroboost unit years ago on a Bronco at Vintage Broncos in Mesa, AZ(remember them?).

At the time, Dean Schaefer(Vintage owner) told me hydroboost units would “send you through the windshield”, which I interpreted as a compliment to their performance. I became somewhat familiar with the hydroboost units through my braking systems research in the intervening years. Hydraulic-assisted braking was used on several large luxury cars in the 1970s and 1980s and has been used in diesel pickups and other heavy-duty applications for many years. The most recent convert to hydroboost appears to be the current generation Ford Mustangs.

Hydroboost is a brake booster that adds supplemental pressure to the braking system via pressurized fluid from the power steering pump. Fluid flows from the power steering pump, through the booster, and then onto the steering box. When the brake pedal is not in use, fluid flows through the booster to the steering box. When the brake pedal is depressed, pressurized fluid flows through a spool valve into a chamber in the booster. This fluid exerts a force on a piston in the unit which pushes on the master cylinder and provides assist to the brakes. The booster also has a part called an accumulator in it that stores the pressurized fluid and allows at least one assisted pump of the brakes if the engine quits running.

Although the booster requires some additional plumbing compared to its vacuum counterpart, part of the unit’s appeal has been its compact size. I always believed that a firewall-mounted hydroboost unit would be optimum, but there was always one catch. Every unit I looked at had an external accumulator canister on it that interfered with the valve covers on the engine, or the upper intake plenum on the Mustang SEFI 5.0 engines now common in the early Broncos. I heard of a unit from a 1974 Chevrolet 1 ton truck that had a remote accumulator, but this setup proved impossible to locate.

Finally, in late 2001, thanks to some great detective work from fellow EBML(Early Bronco Mailing List) member Brian Wickert, a suitable candidate was located. This particular booster was used on 1980-1985 Cadillac diesel cars; Eldorados, Sevilles, and others, as well as several other GM cars during these years. The critical difference between this booster and others is its internal accumulator. This critical feature means it will fit within the tight confines of the space created by the early Bronco firewall and the 5.0 upper intake plenum.

Brian quickly created a mounting bracket for his booster and got his truck back on the road for testing. He reported excellent braking results so I proceeded with my conversion in earnest. I quickly located a suitable donor in a local junkyard and purchased it for $20. I decided to use a Chrysler master cylinder with the booster and procured one from a Dodge van. I had used late model Chrysler master cylinders in the past and like them for their lightweight aluminum body and plastic reservoir with screw-off caps. They’re much handier to use than the cast-iron slugs used by other manufacturers.

As shown in the photos, the booster fresh from the car has an angled bracket attached to it to mount to the Cadillac’s firewall. Foregoing the hassle of finding the official Cadillac tool for removing the locknut on the backside of the input piston housing, I used a Dremel tool and a chisel to remove the nut Following Brian’s lead, I found the angled bracket flipped 90° gave the perfect offset to keep the booster away from the intake plenum.

Keith Jones from ProtoFab fabricated a plate(shown in photos) that combined the Cadillac angled bracket with a plate that bolts to the firewall. The end of the stock input piston pushrod was cut off and a 3/8-24″ threaded coupler was welded to it. I used a standard master cylinder pushrod(shortened and threaded) to extend the pushrod to the proper length to give the proper pedal height. Modifications required on the master cylinder included slotting the mounting holes slightly to match the booster’s mounting stud pattern and adding a homemade spacer to give the proper engagement depth for the booster piston in the Chrysler master cylinder piston.

Plumbing for the system consists of Aeroquip re-useable fittings and power steering hose to create the correct lengths for both the pressure and return lines.

I added a Saginaw pump with two return lines to keep everything looking clean and uncluttered. Goodridge adapters adapt the GM power steering fittings on the booster to AN fittings on the hoses.

While installing the booster, I removed the adjustable proportioning valve in the rear brake line and installed one of Dan Press Industries’ LBS valves, which is supposed to function as a sort of an “automatic” proportioning valve. Braking tests since the installation have shown the valve to be ineffective in panic stops. However, I believe I have exceeded the parameters of its design due to my choice of front/rear caliper sizing ratios and will be installing a new Wilwood proportioning valve upstream of the LBS valve in the near future.

The hydroboost braking system has been installed for about a month and a half at this writing and has functioned well. Braking line pressure has gone from 700psi front/400psi rear(w/proportioning valve) with the vacuum setup to 1000psi front/1000 psi rear(w/o proportioning valve) with the hydroboost system. Pedal feel is different from a vacuum or unboosted setup in that the pedal often feels “squishy” when the pedal is fully depressed. It doesn’t seem to come to a “hard stop” like the other systems do. This may indicate I have air in the lines which will require more bleeding. The “squishy” feeling is offset, though, by the much lower pedal pressure required to bring the truck to a stop in a hurry. Rear lockup is also a problem, but will be addressed with the proportioning valve and other upgrades to be addressed in a future article.

Another issue is the angle of the booster to the firewall. I have been told by at least one source that this 8° angle will cause premature wear on the input piston seals. According to this source, I should have seen power steering fluid pouring out at my feet within minutes of initial startup, but I have yet to see a drop. Whether this concern is valid or not, I am currently redesigning the firewall bracket to accommodate a 90° mounting of the booster to the firewall.

The swap proved to be very economical. I sold my vacuum booster setup and paid for the entire conversion with the proceeds. Many boosters will require rebuilding prior to use, which commonly runs $150-$200. My $20 junkyard special is holding up just fine at this writing.

Technical Sources

Firewall Plate Fabrication & Welding

ProtoFab, Inc.

20295 Conestoga Dr.
Cordes Lakes-Mayer, AZ 86333.
Phone: 520-632-0216
Website: www.protofab4x4.com
Machining, Threading, etc.
We-Be Machining
4520 W. Bryce Lane.
Glendale, AZ 85301
Phone: 602-284-7456.

Firewall Plate Fabrication & Welding

Aeroquip hoses and fittings
P.O. Box 81906
Lincoln, NE 68501-1906
Website: www.aeroquip.com

Hydroboost Unit

Ecology Auto Wrecking
Various locations
Southern California/Arizona
Website: www.ecoparts.com

Saginaw power steering pump, power steering fluid, misc

Local Autoparts store
Your local auto parts

About Rick Webster

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