Vacuum Brake Booster: Testing and Inspection

Preparation

NOTE: To simplify removal of the vacuum hose / non-return valve, actuate the brake pedal several times before starting work to reduce the vacuum in the brake booster.

M42/M43/M50/M52/S52





- Pull non-return valve (1) out of brake booster.
Insert special tool 34 3 101.
Install special tool 34 3 100 (vacuum tester) between non-return valve and special tool 34 3 101 (adapter).

M40/S50





- Disconnect vacuum hose on non-return valve and install vacuum tester 34 3 100 between non-return valve and vacuum hose.

M41/M51





- Pull vacuum line (1) out of brake booster.
Insert special tool 34 3 101.
Mount vacuum tester 34 3 100 between vacuum line and adapter 34 3 101.

Procedure
1. At the vacuum originally built up, however not more than 0.8 bar, and with the vehicle engine switched off in the released position of the brake booster, the vacuum may not drop by more than 0.02 to 0.06 bar during a test time of 15 seconds.

NOTE: Vacuum higher than 0.8 bar can be reduced to specified value by operating brake pedal.

2. At the vacuum originally built up, however not more than 0.8 bar, and with the vehicle engine switched off in the full braking position (pedal) of the brake booster (approx. 80 to 100 bar overpressure in the hydraulic brake system), the vacuum may not drop by more than 0.05 to 0.1 bar during a test time of 15 seconds.

If specified values are not reached:
- Check hose connections for vacuum leaks.
- Replace vacuum non-return valve.
- Check gasket between brake booster and master brake cylinder (cord ring seal) for proper condition and sealing.
- If specified values are still not reached, the brake booster must be replaced.