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MG MGB Technical - brake failure!

Yesterday, when trying to catch the last rays of summersun I had a heart-stopping experience.
I drive a 1970 MGB with dual circuit brake master.

While appoaching the trafic-lights I pressed the brake-pedal and it whent right down to the floor with no brakingpower at all. I managed to pump the brakes back up and used the handbrake in addition. After pulling to the side, I opened the bonnet to check the fluid-level in the brake master cylinder only to find it full. I crawled unter the car to check if there were any leaks, but didnt find any. Anyhow the caliper on the passenger side was really hot and smelt terrible.
Now my question. I bought a brand new brake master cylinder three years ago, but didnt replace the calipers. I know that the one on the passenger side had problems with a sticking piston. I must confess that I havent changed the brakefluid since I replaced the brake master cylinder. Could it be that the sticking piston heated the caliper up to the point that the fluid began to boil and this caused the brake failure? Or is it likely for a 3 year old brake master cylinder to fail? I know, better be safe than sorry, but you can imagine that I dont really like the idea of replacing it again. Your comments on this will be highly appreciated.

BTW. The car got its fresh MOT a month ago.

Best regards

Daniel
Cologe, Germany
Daniel Mahr

Both are certainly possible but I'd chase the sticky caliper first.

Mike!
mike!

Daniel
Nasty experience - agree your assessment of boiling fluid as the cause. I certainly would not expect the master to fail at three years - a problem would almost certainly have shown up before.

In your shoes, I would rebuild both calipers and flush through the fluid at the rear as well. (Worth doing every couple of years).

Good luck

Roger
RMW

The pedal should certainly not have gone to the floor with a dual system, that is precisely what dual systems were designed to prevent! If one caliper were sticking and boiled the fluid, it shouldn't have affected the master and other circuit. If you have the mechanical brake light switch, or there is some other obstruction preventing the brake pedal from returning fully, this can block the fluid return hole in the master and so lock the brakes on as the fluid warms up and expands under normal conditions. When this happens slackening a caliper bleed screw causes a spurt of fluid and the brakes to release. I can imagine that on a dual system this could happen to just one circuit i.e. one front caliper if it were very marginal. Removing the pads and trying to push the pistons back, and watching them as you gently apply the brakes, will indicate whether you have a sticking piston. But personally I would be very concerned about the total brake failure (apart from the pumping) you experienced on a dual system. As a matter of interest did the car pull up in a straight line or to one side when you did get some braking after pumping the pedal?

A 3-year-old master is no guarantee that the problem isn't the master, although it certainly shouldn't be.
Paul Hunt

Daniel, make sure you're rear wheel cylinders
aren't leaking and letting air into the system.
I once had what felt like total brake failure on
a midget with dual circuit brakes -- the problem
was in the rear cylinders, not the master.
This was evident only on removing the rear
drums and checking the cylinder boots for
brake fluid.

You may have two (or three) separate problems.
Siezed front caliper, bad master, and/or
leaking rear cylinders.
Ronald

Have you replaced your rubber brake lines? Old ones swell, get mushy, plug up and cause your calipers to lock up.

You may also have problems with your master cylinder. Sounds like you have two issues going one there.

Bryan

Bryan Prindle

I changed my mind. The sticky caliper(s) need to be addressed but it is now time to play the "head off the domino failure" game.

Upon further reflection, I agree with Paul. It is possible your fresh MC took a dump or the seals are not happy. Either way it also needs attention.

While you are at it, replace the flex lines and the Wheel cylinders. Why? Well, the rubber in them is 30+ years old too (unless you remember replacing them recently, also) and they just don't get better with age.

This is your baby, right (not to mention, potentially, your life)? Best it could stop properly, eh? Beside, spending a bit on the parts now is MUCH cheaper than dealing with an irritated Bentley owner after another "incident."

Just my 2-pence.

Mike!
mike!

Many thanks for your replies.
On tuesday evening I wanted to tow the car away with a friend of mine. At closer inspection my friend (an experienced mg-mechanic) discovered that the mechanical brake light switch had come loose and preventig the brake pedal to return fully.
He said the same about the fluid return hole as Paul wrote.

It looks as though this was the cause, as my friend bravely drove the car back to my house and had no problems whatsoever with braking.

As Roger wrote, this was a really nasty exprience and I will give the calipers a overhaul. As Mike wrote I will get a set of new flex lines, although the ones in use are only 3 years old. I think IŽll go for the steal breaded ones. As wheel cylinders cost next to nothing IŽll change them as well. Concerning the MC has anybody experience with rebuilding one of these?

Regards

Danny
Daniel Mahr

The general consensus here seems to be don't take a chance(with your life) with rebuilding your MC...buy a new one. I am not suggesting that your current unit needs to be replaced...only if it does buy new don't take the risk of trying to rebuild it yourself.
Frank

The dual master cylinder in my 1997 Chevy full size pickup failed in just the same fashion at only three years from new! It happens. I now believe that dual-master systems are primarily intended to prevent brake system loss when leaks occur at the wheel cylinders or hoses. The Master can still fail just as you describe...the seals are all the same age, same condition, etc. In my opinion, failure at three years indicates a defective or poor quality rebuild and these should last much longer, at least ten years in normal use.

Cliff


Cliff Maddox

This thread was discussed between 13/09/2004 and 17/09/2004

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