Does 89 VW Cabriolet have a power or vacuum brake booster on it? We changed the front calipers and brake lines and put new wheel cylinders on the back and put a new master cylinder on. The front brakes are still dragging and getting hot. The pedal feels spongy and makes a "sshh sshh" when you push it down. Could a bad brake booster cause this and if so do we need a power or vacuum booster for this model?

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25 Oct, 2017

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The subdued sound of a burst of air is a sign the vacuum servo or booster is functioning as it should. For most people most of the time a power booster and a vacuum booster are the same things - there are such things as various types of "power brakes" but they are in the world of specialised motoring and commercial and industrial vehicles.

I suggest you revisit each wheel brake and ensure they are all as the vehicle manufacturer intended and the front brakes are not dragging. If they are dragging when turned by hand while lifted on a jack something is clearly wrong. Try briefly releasing a fluid bleed nipple - if that improves matters the hydraulic system has residual pressure. If there is no improvement something about the brake isn't correct.

Once you have satisfied yourself everything is as it should be between the master cylinder and the ground you should ensure the brake pedal linkage is correctly adjusted and operating properly and finally before re-bleeding the hydraulics you should ensure a small clearance exists between the brake booster push rod and the master cylinder piston, about 8 to 10 thou inch is about right...
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