Checking the vacuum brake booster

Checking the servo device is performed in two parts: checking the free play of the pedal and testing the correctness of the vacuum brake booster.

A power booster is a vacuum brake booster. When not working, the driver has a high resistance when pressing the brake pedal which is reflected in the reaction speed and braking safety.

Proper free play of the pedal ensures that the piston of the master brake cylinder returns to the end and opens the compensating lines for the return of brake fluid. Insufficient free play can prevent the piston from fully returning and the brake pressure in the wheel brake units from being released completely. Excessive free play is not good either, as it reduces the reserve height of the pedal in the event of a hydraulic system leak.

The free play position of the pedal is secured and there is usually no need to adjust. There are situations when it is necessary to adjust the free play of the pedal: someone has changed the adjustment, repair, or adjustment of the brake pedal, and the pushrod has worn out over time, which leads to increased free travel, or the power booster is changed. Changing the master cylinder alone does not usually require adjusting the free play of the brake pedal, but it is desirable to verify that it is within specifications.

Measure the height of the brake pedal to the floor of the vehicle. Gently press the pedal with your hand until resistance appears and measure the height. The free play of the pedals is the difference between the previous two measured values. If the measured value is not according to the specifications for the given vehicle, proceed to the free play setting.

The second part of the check is direct testing of the power booster. Preparation for the test is performed with the engine switched off. Depress the brake pedal five or six times to remove all vacuum from the power booster. Then depress moderately and hold the brake pedal. While holding down the brake, the engine starts, and if the pedal sinks 2 to 5 cm the power booster works well. If the pedal does not sink after starting the engine, the power booster does not have its function to increase the braking force and it is necessary to perform additional tests to discover the cause of the fault.

Also, if the engine malfunctions after starting the engine while holding down the brake pedal, and stabilizes its operation when the pedal is released, this may indicate a vacuum leak in the power booster. Then check the external and internal vacuum leaks to detect a fault.

To checking the vacuum brake booster, follow these steps:

Brake pedal free play check

Measure the height of the pedal. With a light pressure of the hand, push the brake pedal to the position when strong resistance is felt and measure the new height. Calculate the free play of the pedal and compare it with the specifications. If the data do not agree, specify all necessary actions.

Vacuum brake booster testing

With the engine switched off, depress and release the brake pedal five to six times. Press and hold the pedal. Start the engine. The pedal should sink 2-5 cm. If this is not the case, perform additional tests to detect the fault.