The MGA With An Attitude

At 06:58 PM 6/9/2008 -0600, Russ Fleehearty wrote:
"I rebuilt the master cylinder, and I cannot get it to pressurize or prime."

Start with a review of bleeding techniques in article HT-106
Also see Faulty Parts article FT-053

A common problem with master cylinder rebuild is that the piston(s) may stick in the bore(s) and not return. This is usually not a problem with the clutch, as the clutch provides a substantial return pressure to help return the master cylinder piston. The issue may be with "modern" replacement seals being a bit too large for the bore.

The fix or cure is best carried out on the work bench immediately after the master cylinder rebuild and before installing it in the car. If the master cylinder piston does not return when pushed in, then apply air pressure to the output port to return the piston to normal rest position. Push the input pushrod again to depress the piston full stroke and release. Use air pressure to return it again if necessary. Repeat the process as many times as required until the piston will return freely from the internal spring force only. This may require dozens of cycles of depression and air return to get it to return freely with the spring force only.

If the MC pushrod is too long it may not allow the piston to return fully to the normal rest position against the MC front cover. Solution is to adjust the pushrod shorter to give minimal end float at the rest position. This should provide 1/8" to 1/4" pre-travel of the pedal (pushed with one finger) before you feel the resistance of the MC piston at the pedal.

When the MC piston is returning to rest against the front plate, and the pushrod has a small amount of clearance at rest, then if the hydraulics still does not work you need to resort to bleeding air out of the system.

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