Vacuum Brake Booster: Testing and Inspection

POWER BRAKE UNIT FUNCTION CHECK
(Simple method)





Step 1
1. With engine off, depress the pedal a few times.
2. Start engine.
3. If pedal moves downward immediately after engine starts; it will be an indication for operation of power brake unit.





Step 2
1. Start engine.
2. Stop engine after 1 or 2 minutes.
3. Depress pedal with usual force.
4. If first pedal stroke is long and second stroke is shorter than first stroke, it will be an indication for operation of power brake unit
5. If a problem is found, inspect check valve or vacuum hose for damage and examine installation. Repair if necessary, and recheck.





Step 3
1. Start engine.
2. Depress pedal with usual force.
3. Stop engine.
4. Hold pedal down for about 30 seconds.
5. If pedal height does not change, unit is operating.
6. If there is a problem, inspect check valve or vacuum hose for damage, and check connection. Repair if necessary, and recheck.
If nature of problem is still not found, follow more detailed inspection described in "Method using tester," below.

(Method using tester)





Connect a pressure gauge, vacuum gauge and pedal depression force gauge as shown in figure. After bleeding air from pressure gauge, conduct test as described in following steps.

Notice: Use commercially available gauges and pedal depression force gauge.

a Checking for vacuum loss





Unloaded condition
1. Start engine.
2. Stop engine when vacuum gauge reading reaches 19.7 inHg (500 mmHg).
3. Observe vacuum gauge for 15 seconds. If gauge shows 18.7 - 19.7 inHg (475 - 500 mmHg), unit is operating.





Loaded condition
1. Start engine.
2. Depress brake pedal with a force of 44 ft. lbs. (196 Nm, 20 kg-m).
3. With brake pedal depressed, stop engine when vacuum gauge reaches 19.7 inHg (500 mmHg).
4. Observe vacuum gauge for 15 seconds. If gauge shows 18.7 - 19.7 inHg (475 - 500 mmHg), unit is operating.

b Checking for hydraulic pressure.





Specification





1. If fluid pressure is within specification (vacuum 0 mmHg) when engine is off, the unit is operating properly.





Specification





2. Start engine. Depress brake pedal when vacuum reaches 19.7 inHg (500 mmHg). If fluid pressure is within specification, unit is operating.