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MG MGB Technical - Brake Failure - Help!

Car is a 72 roadster with new master cylinder about 18 months ago. I'm using Castrol LMA brake fluid.

While driving home from work last week I got stuck in traffic on the Parkway. Temperature was in the 90's, traffic was mostly stop and 'slow go'and the car was running fine, temperature just a little above normal. After about 30 minutes or so of this,I suddenly got no pressure on the brake pedal. Pumping it got me enough to stop, but on the next try it was on the floor again. Managed to get it home by pumping well in advance of each stop. Didn't have time that evening to check anything, but the next day pedal pressure had returned to normal. Haven't driven it until today and all appears normal except the brake fluid in the MC is brownish.

Any ideas why brakes would fail and then return like this?

All help and suggestions welcomed as always.

John
John

Maybe overheated? Check to make sure all your wheels spin freely.
John A

Check the level in the master cylinder. I'm sure it's low.

Look for fluid leaking on the inside of the wheels, running down the tires. You may have a leaky wheel cylinder.

Is there brake fluid leaking down the brake pedal? That would indicate a leaky master.

MG brakes don't like to sit for long periods unused. Wheel cylinders get rusty.

Whichever problem you discover, renew the brake fluid (use only Castrol no matter what anyone says) and bleed and adjust the brakes.

Steve

John. Steve's suggestion is the way to start. Castrol LMA has one of the higher boiling temperatures of the DOT 4 fluids. So, the most likely problem is air has gotten into your system (which is fully consistant with your having to pump up the brakes to make them work).

As Steve notes, the first place to check is the master cylinder reservoir. If it is low, you are loosing fluid from the system and air has gotten into the lines. Refill the MC reservoir, pump the brake pedal a number of times (say, about ten), then go and look for leaking brake fluid. My experience suggests that rear wheel bearings are the most common source of leaks, bad connections or bad rubber hoses the next most common. Leaks from the disc brake calipers are somewhat rare as are broken hard lines. Both, however, do happen. An also less common problem is bad seals on your relatively new MC. While this problem, seals going bad, is not uncommon on old systems, or systems which have not been properly maintained, it seldom is reported now with new master cylinders. Do, however, keep it in the back of your mind. Several years ago, there was a lot of bad master cylinders and people were reporting problems within six months of installation. So, it is possible for the MC to fail in 18 months or less from new. Les
Les Bengtson

I do not think that it is air in your system. The reason for this is that occurs when there is only a small time between brake multiple brake uses, but recovers when left to stand for a time. It then occurs next time it gets multiple uses.
This is not an air problem but a classic example of a master cylinder piston being slow to return. The cause is either a broken spring inside the master cylinder or the brake pedal return lever broken, or not fitted. Often it is a problem with both springs as the master cylinder spring can fail if it it also trying to return the pedal lever each time.

Mick
M F Anderson

One of our club members has experienced three master cylinder failures in the past year. All replacements were new and they came from different suppliers. The last was from Proper MG and about three weeks ago when I last talked with him it was ok. I think his car is a 73 and he uses Castrol LMA fluid. So there are still some bad replacement cylinders in the supply chain. Your symptoms do sound like the master cylinder may be suspect. Brown fluid indicates there is moisture in the fluid but it probably isn't contaminated enough to keep your brakes from working. Overheating of the brakes would be unusual in slow stop and go traffic. FWIW, Clifton
Clifton Gordon

Clifton and all: I read somewhere ( a post on the MGB Experience BBS, I think) that the problem was bad seals in some new MC, and that it is possible to fix the new MC with a Beck-Arnley rebuild kit, which is supposed to have a better quality rubber. Can anyone verify this?
I am also interested, because the brand new Lockheed MC I installed last summer had a leak from the front this spring after I had driven the car a few times just out of storage. There was no loss of function or soft pedal, just some leakage. I removed the pedal cover, the pedal linkage and MC push rod and cleaned the whole area very thouroughly. I reassembled it and pumped the brakes dozens of times, test drove the car, and the leak did not reappear, and has not done so yet. But I an keeping an eye on it. I put this down to weepage during storage, and maybe the use of Valvoline Syntech brake fluid.
Andrew Blackley

Thank you all for your comments. The MC was bought from a local supply house. Could be that it was part of the bad lot mentioned and has been sitting on the shelf waiting for me.

I had wondered about the effect of heat but it doesn't sound like that should be a factor.

I think I'll check again for leaks (didn't find any first time through), replace anything that looks suspect and replace the MC with one from one of our regular suppliers.

Thanks again for your help.

John

John

You might also look for weak rubber hoses; perhaps one of those could get hot and soft to the point it would balloon out.
Tom

There have been reports of old hoses beginning to decompose and forming "chevrons" inside. These pieces of rubber act like a check valve and stop the flow of fluid. Then they gradually return to a normal position as fluid slowly bleeds past.
Dan

This thread was discussed between 29/06/2003 and 30/06/2003

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