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MG MGB Technical - Have to pump brakes

I have a 1973 MGBGT and have suddenly developed a brake problem. The pedal goes right to the floor with no resistance and I have to pump it a couple of times to get any brakes at all. I have bled the system right through and replaced all the old brake fluid but still cannot get a significant improvement. The master cylinder was replaced about 6 years ago and the only thing I have done before the problem started was to replace the brake pressure switch, although I have done this several times before without problems. I do not have a ramp to check underneath, but I cannot detect any obvious leaks and following a brief trip up the road to test it do not appear to be using (or losing brake fluid). Any ideas would be most welcome! Thanks.
Paul
Paul

It sounds like your master cylinder seals are giving up, unless of course it is the remote servo. These have a history of sudden brake failure, but when that happens the master cylinder has usually been emptied into the servo body.
Mike Howlett

Just as an update. I have put the original brake pressure switch back in on the grounds that the problem didn't arise until I changed it, bled the brakes and also used Paul Hunt's trick of getting someone to stand on the brakes while I opened each bleed nipple briefly to expel any air pockets. Brakes are improved and the car is certainly driveable now, although the pedal travel is still quite long. The front pads are pretty low though, so I will replace them and see what difference that makes. The adjustment on the rear brakes however is fine.
Paul
Paul

If you haven't had to add fluid at any time then you don't have leaks. If you have then check fluid isn't running down the pedal into the footwell, the master seals can leak that way and not put any fluid on the ground. Another cause of invisible fluid loss (if you see what I mean) is faulty seals in the remote servo which burns the fluid until you have none left, and no brakes!

Personally I'd be very concerned if this just started happening, and bleeding seemed to cure it, you can almost guarantee it will happen again, and next time you may only discover it when an accident is unavoidable. If the insurance company then happened to find out you had been having trouble with the brakes you almost certainly wouldn't be insured, despite the trauma of causing damage and injury to third parties. But if it only happened after replacing the brake switch then I'd be less concerned, even though that shouldn't introduce that much air. If it's just a matter of the pedal 'pumping up', and it doesn't slowly sink if you apply continued pressure, then the system itself should be OK. But if it sinks you have a leak, which if it is the primary master seal won't cause fluid loss. If it's not now pumping up, but is still a bit long, then that can be caused vby incorrect adjustment of the rear shoes *and handbrake cable*.

Pad wear shouldn't make any difference to pedal travel as the calipers are self-adjusting, it is rear shoe wear that causes a long pedal.

The other question is how come you have so much experience changing switches! If this is on the same car then unless you can find new old stock currently available switches are frequently rubbish and you will have to fit a relay to take the load of the lamps off the hydraulic switch, and you will also have to fit a quenching circuit to the relay as without even that will be enough to blow the switch.
Paul Hunt

Paul H - to answer your queries:I haven't had to add fluid in the normal course of events, only while I was bleeding them. There does not appear to be any loss otherwise and no brake fluid on my shoes! The brakes don't sink when continual pressure is applied, but the firmness of the brakes does improve with a second pump. As to the brake pressure switch, I would say I have had to replace the switch at least once a year. Whether that's because they are poor repros or not though I couldn't say. Point noted on the rear brakes however - I have checked each wheel adjustment, but not the cable adjuster. Will try and do that tonight!
cheers
Paul E
Paul

Paul Leads - When you say you have had to replace the "brake pressure switch", I assumed that you were referring to the pressure differential switch that warns of a leak on one side or the other of a dual brake system. Paul Hunt obviously assumes that you are talking about the brake light switch. Which one of us is correct? If Paul H is correct, then you need to follow his advice and install a relay/arc suppression circuit so the switch doesn't fail repeatedly - see the article, Brake Light Relay in the Other Tech Articles on my web site at: http://homepages.donobi.net/sufuelpumps/ for information on making and installing the circuit. Also, there should never be a need to bleed the brake system after changing out the brake light switch as the master cylinder is higher than the switch, which only results in the loss of a small amount (on the order of a couple of drops)of fluid while making the swap. On the other hand, if we are talking about the pressure differential switch that has been changed repeatedly, then I would suspect a problem in the master cylinder. You could have one of the primary seals allowing fluid to leak around it, but not escape to the outside world. I had that happen recently on our MGB. I found that while the pedal didn't drop when I held constant pressure on it, but if I released the pressure just a bit (not releasing the pedal all the way, just releasing the pressure a little) and then pressing down again, the pedal would travel a fraction of an inch further down. I could actually run the pedal clear to the floor by releasing pressure and stepping down hard repeatedly. I pulled the M/C and found the primary seal to be excessively worn. Replacing the seal and lightly honing the bore cured the problem. Cheers - Dave
David DuBois

Dave - Sorry for the confusion - it's actually the brake light switch! I will check out the article on your web site. Many thanks to all for the input!
Paul
Paul

David - UK didn't get a split system until the 77 model year, hence it wasn't confusing for me :o)

If a second press of the pedal gives a higher pedal then that indicates there is still some air in the system somewhere. It depends on how different the first and second presses are, if the first one gives you palpitations then, perhaps further bleeding would be advisable :o) The remote servo can be a cause of trapped air, as in the MGB the air valve is uppermost and the main cylinder is almost horizontal, giving two places where air cannot escape as they are above the exit point of the cylinder. There is a document around showing that Lockheed recommend the servo is fitted with the air valve pointing downwards and forwards instead of upwards and forwards, and the cylinder is tilted upwards quite markedly. Having said that the second stage high pressure bleeding has always worked for me, as long as the person on the pedal is pressing down on it as hard as they can.
Paul Hunt

Did UK 73 MGB's have a brake servo? The NA models didn't.

Barry
BJ Quartermaine

The remote servo was optional on single circuit systems for most of their use, although it became standard in the UK in 1973. NA had dual circuits from late 67 onwards and there wasn't a servo available for that until the master was replaced by one with an integral servo for the 75 model year in NA, which the UK got for the 77 model year.
Paul Hunt

This thread was discussed between 25/07/2009 and 28/07/2009

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