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MG MGB Technical - Another soft brake issue

Hi All

My 67 GT has a soft brake pedal. If I hit the brakes a time or two, they get hard. I know this is most likely a bleeding issue, but after I bleed it the brakes stay hard for a coupe of days and then go soft again.

Brakes were totally rebuilt including master cylinders rear cylinders and new front disk calipers. This is the original single line system.

I am running silicon brake fluid and I know that can add to the bleeding issues.

Car stops ok, but really like the hard pedal feel. Any thoughts on what is going on and how to fix it?
Bruce Cunha

As well as air in the system the other cause of this problem can be rear brake adjustement. If there is too much movement of the shoe before it engages with the drum you will get this first press soft second press firm symptom.
David Witham

Soft as in spongy? Also long that hardens and shortens with a couple of quick pumps?

Having done brake work on several MGBs I have always found that bleeding needs to be a two-stage process. Initial low-pressure bleeding by either pedal or Eezi-Bleed has always left a long/soft pedal that pumps/hardens up. The second stage is to get someone to press down on the pedal as hard as they can while I rapidly open and shut each caliper nipple in turn, that has always blasted an extra lump of air out even though there were no more bubbles with the low-pressure bleeding. After that the pedal has always been fine.

I think incorrect rear shoe adjustment causes a long pedal that can be pumped up, but it shouldn't cause a soft/spongy pedal.
PaulH Solihull


Paul that is a pretty good description of the issue. I will give your "Second" step a try.
Bruce Cunha

I have a '67 as well and think that the rear brake adjustment would warrant a second look if Paul's suggestion doesn't cure the problem. I just replaced the entire braking system on my car, a single circuit system also, and the brake pedal has a total movement of less than 3/8" and is rock hard. This is with Castrol LMA DOT4 however. RAY
rjm RAY

This thread was discussed between 29/07/2010 and 30/07/2010

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