Master Cylinder Sticks after Re-Build

Some owners have experienced problems with their clutch and/or brakes after rebuilding the master cylinder. This is often caused by the failure of the piston to return properly to the start position after use. The clutch is less prone too stick because the springs in the cover generate a stronger return pressure. However, the problem may still occur when the system is being bled, because until the system is filled with air-free fluid, the return pressure cannot be transmitted

The problem may be caused by after-market rubber parts being made slightly over-size or simply because the new material needs to bed in effectively against the side of the bore before they will move smoothly.

Naturally the cylinder should be rebuilt "wet" with plenty of brake fluid being used as lubricant. Once the cylinder internal parts have been reassembled, but before fitting the gasket and end-plate use a convenient rod, such as a piece of dowel or a pencil, to push the piston down the bore. It should pop back under its own back-pressure. If it does not, you need to work it back and forth until it frees up. Easier said than done, because if it has stuck at the bottom of its stroke it is a swine to get out again. The answer is to jury-rig an air line to the pipe connection and use air pressure to return the piston after each stroke. Be careful with compressed air or you may have created a lethal large-bore air-gun. A foot pump is better. While you are bedding in the rubbers, it is sensible to remove the non-return valve parts at the bottom of the bore but do not forget to replace them afterwards.

Once it is moving freely, the end plate can be fitted and everything reassembled into the car.

See the separate article on bleeding techniques for relevant tips.

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