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MG MGB Technical - brake failure

hi guys
got into my 76 bgt daily driver this morning and luckily only doing 10mph went to brake and pedal had no resistance and went straight to the flour. as most will know freezing last night don't know if that helps. but where should i start in terms of hunting down the problem to sort it out.
jr williamson

John, you were lucky you weren't doing 70 down the motorway. The cold weather is nothing to do with it. Have a look in the master cylinder. I bet its empty. If it is, the fluid is almost certainly all inside the servo. This has happened to many owners of cars with the remote servo. The seals in the servo fail, but you never see the leak because the fluid just collects in the body of the thing. Only when there is no fluid left do you find out, sometimes with disastrous consequences.
Mike Howlett

if that is the case is it a seal replacement or new servo. if so is it a hard job to do.
thanks john
jr williamson

It could be a leak anywhere at all in the system. Have a look on the inside of the wheel rims, if fluid is leaking from the caliper than it usually collects here. Otherwise you'll need to check every pipe and connection for leakage.

BTW, rubber seals and rubber pipes are affected by the cold. But it wasn't that cold!


i don't know about it not being that cold there was a nice thick layer of ice on the car.
jr williamson

Neil's right of course, it could be leaking anywhere, but the most likely, after the servo, is the rear wheel cylinders. You might see the fluid stains on the rear of the back brakes, but really you need to remove the rear wheels and drums to see if everything inside is covered in fluid. If the master cylinder has leaked, you will find the fluid under the carpet behind the brake pedal. Personally, in over 40 years motoring and DIY maintenance, I have never seen a leaking front caliper. I've seen plenty of stuck ones, but never a leaker. I still favour the servo.
Mike Howlett

Neil didn't say it wasn't cold, he said the cold was nothing to do with the failure. Cold weather doesn't make brake systems fail of course, although it's just possible somwething in your system was so marginal the extra hardening and lack of flexibility of the low temperature *was* just enough to make the difference i.e. in rubber parts somewhere. A 76 should still have the remote servo, which won't in themselves cause a flooring pedal unless they have sucked all the fluid out of the system, which is a known failure mode. Incidentally it burns the fluid as much as collecting in the body. But you won't know where to start spending money until you have investigated where the fluid has leaked from if it *has* leaked. If the master is still full then it is the master seals that have failed.
Paul Hunt

In the last 10 years,with 2 cars,I've had leaks from LH caliper, both rear cyls, master cyl, remote booster & all 3 flexible pipes including braided ones. So take your pick ! Barrie E
Barrie Egerton

This talk of the brake servo has reminded me that I need advice about the air filter on top of the servo. Why does it rotate freely, and can I remove it to paint it. How far can one go in removing the filter without getting into brake fluid? Hope that John and others don't think this is too far off the subject. Many thanks
MP Alston

Why not? It doesn't do any harm either way, there is nothing to be gained by making it non-rotating. You can remove the screws to remove the larger air-valve cover, diaphrgam, diaphragm support and valve housing, then below the joint washer you have the piston,piston cup and slave cylinder and are into fluid. If you are going to paint the servo body - and really it should be bright coated and not painted, definitely the remote servo if not the integral one - I'd loosen the screws to enable masking-off of the valve housing and not completely remove it.
Paul Hunt

Many thanks Paul, that's very helpful.
Maurice Alston

This thread was discussed between 31/10/2008 and 02/11/2008

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