Technical Information - Braking
Vacuum Brake Servo - Test Procedure
The servo has a reservoir which stores vacuum either side of a diaphragm, application of the
brake pedal allows air to enter the rear chamber and assist the driver’s effort in applying the
brake. The vacuum is drawn from the inlet manifold on petrol engine vehicles and a pump
on diesel engine vehicles.
No assistance generally means;
A:
B:
C:
D:
A failed vacuum pump (diesel).
A failed non return valve.
A leak from the vacuum pipe or servo.
Collapsed vacuum pipe (not visible from external examination)
Replace any of the above as required.
To Test;
Test 1
Test 2
Test 3
Exhaust the vacuum by pumping the brake pedal several times without the
engine running until the pedal travel becomes short and hard.
Depress the brake pedal with reasonable pressure and start the engine, the
brake pedal travel should increase as the vacuum builds up.
Disconnect the vacuum pipe and connect to gauge and check for vacuum
with engine running (minimum 0.5 bar negative pressure) Low or no
vacuum, check pipe, connections and pump (diesel), replace as required.
On some diesel engine vehicles, with the engine running if prolonged pressure is applied to
the brake pedal the vacuum pump can build up sufficient energy to generate excessive
pressure in the brake system resulting in long brake pedal travel. This can fool the operator
into thinking there is a problem.
As a safety check; exhaust the servo of vacuum as in test 1. Depress the brake pedal and hold
for several minutes, if there is a hydraulic leak within the system, the pedal travel will
increase without the servo assistance - Retest as test 2.
Please see over for diagram of a Vacuum Brake Servo.
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October
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2004
Technical Information - Braking
Vacuum Brake Servo - Diagram
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TECHNICAL HELPLINE
01926 472794
October 2004