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Disk Brake Booster - Drain Grommet Removal
BPW disk brake boosters are different to that of drum brake boosters. The service section of the disk brake booster contains a bellow as indicated in Figure 1. This bellow acts as a seal to protect the disk brake calliper from moisture and dirt. The front of the service section has a rubber O-ring, which is part of the bellow (see Figure 2) and seals the unit when it is tightened to the disk brake calliper.
In addition the service section is sealed off to the rest of the brake booster via the diaphragm, see Figure 3.
Importance of removing the drain grommets
Once the brake booster is installed onto the disk brake calliper, it is very important for the drain grommets to be removed from the lowest drain hole, Figure 4. All other grommets must be kept in place.
When the brake is actuated, compressed air is applied to the primary chamber, Figure 5, of the air cylinder. The diaphragm tries to displace the air from the secondary chamber. If the drain grommets are still within the drain holes the air only escapes very slowly and an over pressure builds up in the secondary chamber. Once the air has escaped from the secondary chamber and the brake pedal is released, a vacuum is created and the piston rod of the cylinder remains in the extended position since the force of the resetting spring is no longer sufficient to push the membrane back.
Correct removal of drain grommet
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