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The Hydro-Boost system, Figs. 1 and 2, provides an additional cylinder in the brake system. This cylinder contains no brake fluid. The Hydro-Boost cylinder is hydraulically operated, by pressurized fluid from the power steering pump, providing power assist to operate a dual master cylinder brake system.
The booster is composed of two sections; the linkage section and the power section.
The booster is designed so that if a total absence of power assist occurs, the brakes can be applied manually, but somewhat greater pressure on the brake pedal is required.
Testing and Inspection
Complaints about power brake operation should be handled as if two separate systems exist. Check for faults in the regular brake system first. If it is okay, start looking over the power brake circuit. For a quick check of proper power unit operation, press the brake pedal firmly and then start the engine. The pedal should fall away slightly and less pressure should be needed to maintain the pedal in any position. On vacuum suspended power units, air will rush into the air intake when the brakes are applied.
Another check begins with removal of the stop light switch and installation of a pressure gauge. Take a reading with the engine off and the power unit not operating. Maintaining the same pedal height, start the engine and take another reading. There should be a substantial pressure increase in the second reading.
Pedal-free travel and total travel are critical. As a general rule, brakes should be adjusted or relined if the pedal is closer than 1-1/2 in. from the floor with the brakes applied. Free travel should be kept strictly to specifications.
Take a manifold vacuum reading if the power unit isn't giving enough assistance. Remember, though, that some of the new V-8s have less than 15 in. of vacuum at idle. If manifold vacuum is abnormally low, tune the engine and then try the power brakes again. Naturally, loose vacuum lines and clogged air-intake filters will cut down brake efficiency. Most units have a check valve that retains some vacuum in the system when the engine is off. A vacuum gauge check of this valve will tell you when it is restricted or stuck open or closed.
A simple check of the hydraulic system should be made before proceeding. Loosen the connection between the power unit and the wheel cylinder lines. If the brakes release, the trouble is in the power unit hydraulic circuit. If the brakes still will not release, look for a restricted brake line or similar difficulties in the regular hydraulic circuit.
A residual pressure check valve is usually included immediately under the brake line connection on hydraulic-assist power brakes. This valve maintains a slight hydraulic pressure on the brake lines and wheel cylinders to give better pedal response. If it is sticking, the brakes may not release.
Power brakes that have a hard pedal are usually suffering from a milder form of the same ills that cause complete power unit failure. Collapsed or leaking vacuum lines or insufficient manifold vacuum, as well as punctured diaphragms or bellows and leaky piston seals, all lead to weak power unit operation. A steady hiss when the brake is held down means a vacuum leak that will cause poor power unit operation.
Do not immediately condemn the power unit if the brakes grab. First look for all the usual causes, such as greasy linings or scored drums. Then investigate the power unit.
Bleeding Booster - Method #1
1. Fill power steering pump until fluid level is at base of pump reservoir neck.
2. Disconnect 12 volt wire from injection pump or distributor, then crank engine for several seconds. Do not start engine.
3. Check fluid level, adding as necessary.
4. Connect 12 volt wire and start engine.
5. Turn steering wheel from stop to stop, then turn off engine.
6. Depress brake pedal four or five times to deplete accumulator vacuum.
7. Check fluid level, adding as necessary.
8. Start engine, turn steering wheel from stop to stop. Turn engine off, and check fluid level.
9. If fluid is extremely foamy, stop engine and let vehicle stand for one hour, then check fluid level, adding as necessary.
The method below is the one I use - with the front wheels off the ground.
Method #2:
Bleed hydraulic system as follows:
1 .Fill power steering reservoir to proper level and allow vehicle to sit for approximately two minutes.
2. Start engine and allow to run for several minutes, then shut engine off. Correct fluid level as necessary. Do this several times until fluid level becomes constant.
3. Raise and support front of vehicle, then turn wheels lock to lock. Shut engine off and add fluid as necessary.
4. Lower vehicle, then start engine and depress brake pedal several times while turning steering wheel lock to lock.
5. Shut engine off, then depress brake pedal 4 to 5 times to allow accumulator to deplete.
6. Check reservoir and refill as required. If fluid is extremely foamy, allow vehicle to sit for several minutes, then repeat procedure above.
Note step 3: where it says to stroke the steering stop-stop while the wheels are up.
While the GM procedure only suggests that this be done one time, I do it repeatedly. It is much easier to purge the air out of the piping with the tires free to move - simply because it is easier to spin the steering wheel.
Also, it is critical that close attention is paid to the level in the P/S reservoir during this step. While an assistant strokes the steering wheel, I watch the level. Have the helper move the slowly at first and then more quickly as the amount of bubbling diminishes.
Have a bottle of power steering fluid handy and keep adding fluid to replace the volume of air that will be purged from the hydraulic system - it will bubble up through the liquid in the reservoir. If the level drops too low, the steering gear will start drawing in air - rather than hydraulic fluid - and force it into the discharge piping. The entrained air will cause the steering/hydro-boost systems to develop air pockets and the procedure will have to be repeated.
One last note:
If, as is in your case, the steering system reservoir is remotely mounted (as shown on the right in the diagram below) be prepared to spend extra time purging the system. This is due to the increased volume of the piping - as opposed to a pump mounted reservoir, which basically has no piping other than the lines to and from the hydro-boost unit. If you also have a P/S cooler installed - it can really take some time to get all the air out.
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