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MG MGB Technical - Brakes failing

1964 MGB. Brakes are fine unless I rapidly pump on the pedal. They are not spongy when I first press the pedal. However, if I keep my foot on the pedal, let up just a bit and push again, the pedal will go down further each time until it eventually goes to the floor. If I let the pedal come up completely before pushing again, the brakes work correctly. Not losing any fluid. Doesn't make any difference if it has been running or not or if the handbrake is set. I am guessing a master cylinder problem, but was wondering if there is anything else I should be checking before pulling the master cylinder.

Thanks for any suggestion.
Richard Donnerstein

It's almost certainly the seal on the master cylinder failing, contrary to popular belief the master cylinder does not usually loose fluid when the seals begin to fail. I had a similar problem recently when my car developed what I now know is called "long pedal". That's where the brake pedal will slowly sink to the floor as you hold your foot on it. This is apparently caused as the fluid seeps past the main seal yet is retained within the cylinder - hence no loss.

From my experience all rubber seals on brake parts will deteriorate more quickly if the car is not used for long periods, regular use helps keep the rubber flexible.

Don't leave it too long to fix - you're heading towards the point of loosing ALL your brakes especially in the event of an emergency stop.


Mike M

This thread was discussed on 19/06/2005

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