Checking Complaints

Complaints about power brake operation should be handled as if two separate systems exist. Check for faults in the hydraulic system first. If it is satisfactory, start inspecting the power brake circuit. For a quick check of proper power unit operation, press the brake pedal firmly, then start the engine. The pedal should fall away slightly and less pressure should be needed to maintain the pedal in any position.

Another check begins with installation of a suitable pressure gauge in the brake hydraulic system. 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 on cars equipped with power brakes. Pedal travel should be kept strictly to specifications.

Take a manifold vacuum reading if the power unit isn't giving enough assistance. Remember, though, currently produced emission controlled engines, manifold vacuum readings may be less than 15 inches Hg 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.

Failure of the brakes to release in most instances is caused by a tight or misaligned connection between the power unit and the brake linkage. If this connection is free, inspect for a broken piston, diaphragm or bellows and return spring.

A simple check of the hydraulic system should be made before proceeding. Loosen the connection between the master cylinder and the brake booster. If the brakes release, the trouble is in the power unit; if the brakes still will not release, look for a restricted brake line or similar difficulties in the 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 within the brake lines and wheel cylinders or caliper 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, scored rotors or drums. Then investigate the power unit. When the trouble has been traced to the power unit, check for a damaged reaction control. The reaction control is usually made up of a diaphragm, spring and valves that tends to resist pedal action. It is put in the system to give the pedal "feel."