Hydroboost Power Brakes 101 | How to Bleed Hydroboost Brakes

When it comes to vehicular braking systems, there are three main types—manual (non-power assist), vacuum brake booster assist, and hydraulic brake booster assist. Few vehicles use manual systems anymore, but many classic cars still have the original manual system installed, and manual brakes were still available up until around 1990. Vacuum-assist braking systems are the most common and are still used on most vehicles sold today. These systems are simple and easy to work on. However, there is one more style that is less known, the hydroboost brake system.

Manual brakes typically generate 700 to 1,200 psi of braking pressure, depending on the pedal ratio and master cylinder diameter, but they require significant legwork to get there. Vacuum braking systems develop 800 to 1,400 psi in most cases with significantly less pedal effort than manual brakes. Hydroboost brakes require much less pedal effort than vacuum brakes while upping the total braking pressure into the 2,000-2,500 psi range. Your braking power is substantially better with a hydroboost brake booster than with any other system, and they are not dependent on your engine building enough vacuum.

When you depress the brake pedal, the input rod actuates a lever that moves the spool valve, which allows the pressurized fluid from the power steering pump to flow through the power chamber. The pressure in the power chamber pushes on the output rod, moving outward and actuating the master cylinder piston. When the brake pedal is released, the spool valve closes, allowing the fluid to pass through the hydroboost brake booster unit and on to the steering gear. When in use, the fluid still travels to the steering gear at the same pressure. The operation is seamless, you never feel anything funky from the brake pedal apart from at start-up. When you first start the engine, you will feel the brake pedal push back a little bit on your foot. This is the system engaging. Think of it as cocking a hammer on a gun, your brakes are now ready for action.

In the event of an engine failure, pump failure, or broken line, you still have brakes. All hydroboost brake booster systems have a backup pressure canister called an accumulator that stores pressure. Most systems give you one or two brake applications at 75 percent of the normal operating pressure, another one or two brake applications at about 40 percent pressure, and then minimal pressure after that. Once the backup pressure is expended, you can push through the hydroboost just like you can with vacuum brakes, it just requires more effort, as it will technically be applying pressure like a manual brake system at that point.

Hydroboost Brake Bleeding | Vacuum or Pressure Bleeding

This is the preferred method for how to bleed your brakes, as it eliminates all the air in the system. These methods eliminate the need to operate the brake pedal entirely since you use a special tool to push or pull fluid into the system. Pressure bleeding is usually done at the master cylinder, using a pressure pot and master cylinder lid adapter to push clean, fresh brake fluid into the system with each bleeder opened one at a time from the furthest away to the closest to the master cylinder.

Vacuum or pressure bleeding uses a brake bleeder that generates vacuum or pressure, depending on the design. Vacuum pumps pull fluid and air out of the brake bleeders, drawing fresh fluid from the master cylinder. Whereas pressure bleeders apply pressure to the master cylinder, pushing fluid through the system and out the bleeders at the wheels. It is imperative that you regularly check the fluid in the master cylinder reservoir during the process to ensure it does not run dry, which allows air to enter, and you have start over. These pumps work well and are compatible with all types of brake fluid. The vacuum pump tool is also extremely helpful for any service or testing that involves a small amount of vacuum.

A different form of bleeding that works best on new systems or well-maintained braking systems is reverse bleeding. Instead of pushing fluid from the top, this type uses a hand pump like the vacuum pump tool, but instead pushes fluid from the caliper/wheel cylinder up to the master cylinder. This is the cleanest option, as there is no mess to clean up provided you don’t overfill the reservoir. This also eliminates any air in the master cylinder or combination/proportioning valve. The Phoenix reverse brake bleeder works with virtually any braking system and is easy to use for hydroboost brake bleeding. The one issue with this form of brake bleeding is that if your lines are old and corroded, you may push any residue in your lines into the reservoir instead of out of the system. In the case of a brake line flush (which should be done every 2-3 years), flush from the top before refilling the system with a reverse bleeder.

Manual Bleeding a Hydroboost Brake System

If you don’t have one of the brake bleeding tools just mentioned and need to bleed your brake system now, don’t fret, you can certainly still use the classic two-man bleeding system. You just need a little more pedal effort to accomplish your hydroboost brake bleeding. If you have a lift or jack stands, you can run the engine while bleeding the brakes. There is a major safety caveat- do not run the engine while bleeding the brakes without ensuring there is zero possibility of the drivetrain engaging. One person will be under the vehicle, so there is a very real possibility of a crush injury. This is the main reason for using one of the tools described above. Safety is first, always.

GM Hydroboost Bleeding Procedure

The hydroboost itself in most cases is self-bleeding, however there is a procedure you can use if your hydroboost braking system is making noise. Air can be trapped in a hydroboost, particularly in a new installation or after replacing a pump, line, or steering gear. The process requires raising the front of the vehicle until the front wheels are off the ground. It can be done on the ground, but it is much easier and faster when they are lifted. With a full power steering reservoir and all lines tight, start the engine with the steering centered. Slowly turn the steering wheel about a half-inch from center in both directions (left, then right or vice-versa), then do it again but an inch off center in each direction. Continue this process until you reach lock to lock, adding a half-inch each time. This is a time-consuming process. Regularly check the fluid reservoir to ensure it is full. If you see bubbles, stop, shut off the engine and let them settle before restarting the entire process. If the bubbling continues after several tries, you are likely to have a leak in a line or fitting.

The other key is a well-maintained power steering system. Your pump needs to be up to the task of handling both the hydroboost booster and the steering gear. If the pump is in good working order, it should do the job, however pressure reducers or low-pressure power steering pumps may not have the juice (pun intended) to handle both systems in operation at the same time, such as applying the brakes while turning the steering wheel.

These conversions require additional hydraulic lines, which may be expensive to build. Braided stainless steel Teflon line kits are available but difficult to assemble. Any hydraulic line capable of handling 3,000 psi is suitable for the pressure lines. The return lines often require a tee fitting to allow used (de-pressurized) fluid to return to the reservoir quickly without creating a restriction, Y fittings are best for this. If possible, using a power steering fluid reservoir with dual return line fittings is the best option. Additionally, your system will use a little more fluid, if you have a remote reservoir, your main feed line may need to be upgraded to facilitate the increased flow requirements. A minimum 3/4-inch line is recommended for remote reservoirs to feed the power steering pump.