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MG MGB Technical - Brake fluid loss

It has been a few months since I took my 70BGT out for a little exercise. When I depressed the brake pedal it was low and spongy. I checked the reservoir and found the rear portion of the master brake cylinder nearly empty. Assuming there was a leak in one of the rear brake cylinders I removed the wheels and drums - everything was perfectly dry. When the master cylinder leaks it would run down the brake pedal - it too was dry. I added fluid, pumped the pedal and it returned to firm pressure and normal brake height.

I have never had brake fluid evaporate. Does anyone have an idea where the fluid went?

LS Sheldon

Lee. You do not mention how long it has been since you last checked the level of the master cylinder. As the pads wear in front, some brake fluid is moved from the master cylinder to the brake calipers. The same thing happens as the shoes in back become more worn, especially if you do not adjust the shoes on a regular basis. These are the good ways to have the master cylinder reservoir lose fluid.

The bad ways are a tear in one of the fluid seals on the calipers or a tear on one of the seals in a wheel cylinder. (Signs of fluid in the area are the best clue, especially with a wheel cylinder which will fill the brake drum with fluid.) Other areas of loss are any flex hose or any junction point in the system. You need to clean and inspect all of those points from the hard lines attaching to the master cylinder, down to the junction on the inner fender, then along the lines to where they attach to the flex hoses. At some point, you should find the point of loss if there is some form of loss to the external environment.

Also, check the bulkhead that the pedal box and master cylinders is attached to. A leak at the rear of the master cylinder may drain down the pedal and show up on the floor, or it may drain down onto the bulkhead and not show up on the floor.

Les Bengtson

Lee - I am looking at your address and thinking warm and sunny and at Les' advice. Sometimes the holes in the system are so small that the leakage can get washed away by rain water (but perhaps not in California). You may just have to move the lines away from the bodywork in order to find them. I hope it turns out easier than my GT last year.
Roger W

Lee: I had the same thing happen on my 79 B. There no signs of leakage but it emptied out every time I drove it. Come to find out the M/C rear seal failed and the fluid was being sucked through the vacuum booster into the engine along with the fuel and combusted. Replacemnet of the M/C solved the problem.



79 MGB
gary hansen

Les makes an oblique reference to the fact that the unboosted split system master feeds the front brakes from the rear section, and the rear brakes from the front section. So when the rear section is low, it is the front circuit you should be looking at, not the rear.

Gary's problem can only happen to the 77 and later boosted split master of course.
Paul Hunt

I had a customer complaining about oil leaking on his garage floor. No oil leaks were found but the master cylinder was almost empty. The brake pressure warning switch had decided to leak while setting quietly.
ss sanders

I did a complete check of all of the brake lines leading to the cylinders as well as checking out the master cylinder and all wheel cylinders - everything is nice and dry. I guess the fluid was not lost but just filled the cylinders allowing for wear on the pads and shoes. Needless to say I will continue checking.

And yes, the rear cavity in the master cylinder does feed brake fluid to the front brakes.

Reply to Roger in the UK. RAIN what's that? Our annual rainfall is around 20" per year but 2013 was the driest on record. Our reservoirs are filled from the snow fall in the Sierra's but there has been no snow. Everyday is clear and sunny with average daily temperatures in the high 60 degrees F.

LS Sheldon

This thread was discussed between 03/01/2014 and 04/01/2014

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