With the arrival of the Volkswagen Bay Window Bus, it was possible to equip your car with power brakes. At the beginning, the sales figures of the VW Bus with power brakes were quite low, the reason being the extra price. Later this changed and by the end of production there were more Buses with than without power brakes.
In the brake booster there is a diaphragm with negative pressure on both sides. As soon as the brake pedal is pressed, a valve opens and lets in the pressure of the outside air. The resulting lower pressure at the front of the diaphragm helps push the piston into the master cylinder.
You can test the brake booster as follows:
1. With the engine not running, apply the brake 5 times (the vacuum disappears).
2. At the last kick, keep the pedal depressed.
3. Start the engine.
4. If the brake booster is working properly, the brake pedal will drop significantly.
If the brake booster is no longer doing its job, you can still brake, but it takes a lot more force. In many cases, a torn diaphragm will cause the brake to stop, but it's also possible that the problem is a leak in the pressure line to the motor. If the problem is in the brake booster, you will need to replace it completely, repair is not possible.
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