Initial Inspection and Diagnostic Overview

Diagnose power brakes systems as if two separate systems exist. Check for faults in the regular brake system first. If no problem exists in the regular brake system, diagnosis of the power brake circuit is necessary. 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.
To perform another check, remove the stop light switch, then install 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 inch from the floor with the brakes applied. Free travel should be kept strictly to specifications.
If the power unit isn't giving enough assistance, take a manifold vacuum reading. If manifold vacuum is abnormally low, check engine performance, 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 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 could be caused by collapsed or leaking vacuum lines or insufficient manifold vacuum, as well as punctured diaphragms or bellows and leaky piston seals. A steady hiss when the brake is held down is a vacuum leak, which will cause poor power unit operation.
Do not immediately replace the power unit if the brakes grab. First look for all the usual causes, such as greasy linings or scored drums. If the hydraulic system is satisfactory, diagnosis of the power unit is necessary.