Checking - adjusting: Special tool

SPECIAL TOOL

A : Pressure gauge - Vacuum gauge
Reference : 8.1503 A + D

IV - CHECKING THE SERVO BRAKING SYSTEM

With the engine stopped, depress the brake pedal a number of times to « use up » all the vacuum in the brake servo. The pedal will become stiff and will remain in the « upper » position. Keep the foot on the brake pedal and start the engine. The pedal should move slowly down.

As, on diesel engined vehicles, the exhaust-er does not provide sufficient vacuum at idling speed, press down on the brake pedal and accelerate the engine at the same time. The pedal should move slowly down.

Note : If the brake servo is in good condi-tion, there should be no change in the en-gine speed when the pedal is depressed.

CHECKING THE VACUUM

On petrol engined vehicles :

• Connect, to the engine : - a pressure-vacuum gauge 8.1503A+D - a tachometer.

Warm up the engine until the fan cuts in. Accelerate to 4500 rpm. Cease accelerating the engine. The maximum pressure indicated during the time that the engine speed is falling should be equal to or greater than 500 mm/Hg or 700 mbars.

On Diesel engined vehicles :

• Connect a pressure-vacuum gauge into the system.

Run the engine for at least one minute at fast idling speed. The vacuum shown should be equal to or greater than 500 mm/Hg or 700 mbars.

If a vacuum of 500 mm/Hg is obtained, the servo braking system is not responsible for the defect. Look for the cause in the hy-draulic system.

IF A VACUUM OF 500 mm/Hg IS NOT REACHED :

Disconnect the valve (2). Repeat the test whilst blocking the valve end fitting with the thumb.

1st Case : If the vacuum is less than 500 mm/Hg : Look for the cause on the output side of the valve, that is to say :

On Petrol engined vehicles :

• Air leaking into the system : because one of the following is loose or damaged : - the valve (2). - the pipes. - the carburettor securing flange.

The condition of the engine : check the compression pressure or pressures.

On Diesel engined vehicles :

• Air leaking into the system : because the valve or the pipes are loose or damaged.
• A defect in the exhaust-er : Check, initially - the belt tension - the exhaust-er oil lever (too high an oil level reduces the output).

2nd Case : If the vacuum is 500 mm/Hg or more :

• Reconnect the valve (2) after fitting a new seal (1).
• Repeat the test.

If the vacuum is, again, lower than 500 mm/Hg the brake servo is the cause of the trouble. Check the air filter on the brake servo and clean it. Repeat the test.

If the vacuum is too low, the brake servo diaphragm is leaking. Replace the brake servo.