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MG MGB Technical - Brakes failed (as did my nerves) but okay when pumped

Just got back from an hours drive and the brakes failed, luckily on a flat road with no traffic!
I pulled over and pumped the brakes, seemed fine. Set off and braked at first corner, failed again although managed to stop the car after I stamped on brake for the third time (despite a massive screeching!).
Drove home and after a hundred yards brakes are fine, drove all way home successfully. Strange thing is, brakes seem to be better now than before, don't need much effort to stop now.
On another topic I have an oil leak which is burning away on the exhaust but I noticed a much stronger burning smell this time (not just when braking, all the time, if it's relevant).
Brake fluid level seems fine.
Obviously my trust in the car has gone now, does anyone know what it can be?
I'm terrified of it happening at a more crucial time and probably won't drive it again till I can solve it.
Thanks,
Rob
--
1971 MG B Roadster
Rob Silkstone

Most likely cause is a failed master cylinder. (I assume your
car is non-servo, a servo failure can seem like a brake failure.)
Note that the MC seals can fail internally, so you won't see
any fluid leaks, yet the cylinder can't maintain pressure.

Also check for leaking rear wheel cylinders. I found that
a leaking rear cylinder can give similar symptoms as a filed MC.
I would replace all hoses and check for fluid leaks from unions
and pipes as a precaution.
Ronald

Rob: Ronald is spot on. I had the same thing happen on my 69 (scared the hell out of me on a fast downhill bend). I just replaced all the old hydraulics and it works like new.
Andrew Blackley

Many thanks
(I lied by the way, I couldn't leave it alone and been out again and no problem whatsoever!).
I'm away for a few days now and have my ex-mechanic pal ready for an investigation this weekend.
Is replacing the master cylinder an expensive part to purchase?
Thanks again for your help,
Rob
Rob Silkstone

I believe your problem is a sticking caliper, which has warmed up the fluid and began to boil any moisture that it has absorbed. This creates expansion of vapour in the hydraulic system that then requires the brakes to be pumped in order to work. After the brakes cooled down the vapour has turned back to fluid and thus the brakes will be normal again. You suggested that you could smell something at the time which could have been heated brakes, the question is did you notice the car pulling to one side in normal driving? this would be a good sign of caliper sticking!
Bob

my 2 cents...

There's no doubt in my mind that the Master Cylinder has failed internally..Even though you say that it is okay after talking another drive I would replace it asap. I guarantee that it will fail again soon and next time you may not be able to avoid an accident. If you are capable enough, rebuild kits are available for the master cylinder everywhere as long as the cylinder bore is in good shape..

Eric
EJW Willis

Yes!!!!!!!
The car did seem to be pulling to the right, it was so apparent I took her to a straight road I know, road her straight and watched her creep over to the right!
Thanks Bob, will check calipers same time as I check the master cylinder.
Rob
Rob Silkstone

May I add I know nothing of which you (all) speak, I'll be printing this for my pal and learning from the experience!
Thanks all,
Rob
Rob Silkstone

My car also pulled to the right, but it was due to a leaking rear wheel cylinder. With new ones, new fluid, new hoses, new MC, all is now well and braking joy and equilibrium has been restored.
Andrew Blackley

Probably the MC. Often you will notice the brake pedal "creeping" down at a stop light before they start to fail. It was a pretty simple job to rebuild the one circuit MC on my 67, not sure about a dual circuit. The kit was less that $10 if memory serves me correct (but not infallable these days!)

Good luck,

Bud
Bud G

If it was a master cylinder problem you would not have had the smell, if the rear cylinder was leaking you would have run out of fluid!
Bob

What's the remedy for a sticking caliper Bob, simple adjustment/lubrication or a replacement?
Thanks,
Rob
Rob Silkstone

You could pump out the pistons and clean them and the bores and replace the seals, but if they are scored you will end up replacing them anyway. If replacing you really need to do both sides.
Paul Hunt

The proble is that the caliper may not stick again for a while, you must train yourself to be aware of the symptoms so that you know when it happens. If it were me I would put on 2 recon calipers and forget about the problem. Recon calipers are only £35 each so it is not expensive to do the job correctly.
Bob

Thanks for all the assistance.
The problem was a leaking rear nearside brake cyclinder but, as there is slight wear to the front disks I'm gonna renew front pads, disks and rear shoes (plus the cylinder of course!).
The whole lot from MGBHive is £57.00, which certainly won't break the bank!
Thanks again,
Rob
Rob Silkstone

This thread was discussed between 04/06/2002 and 10/06/2002

MG MGB Technical index

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