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MG MGB Technical - Have to pump the brake

I've got a 1979 MGB. It has disc brakes on the front and shoes on the back. Recently the brake pedal can be pushed nearly all the way to the floor and requires a pump or two to get adequate resistance to brake. No fluid is leaking. The reservoir remains full. Would this be caused by an internal master cylinder leakeage or pressure failure switch not working? What do you think?
leeload

Could be a leak in the master brake cylinder. Could also be an air bubble. I'd try bleeding the brakes and see if that works before you commit to replacing hte seals in the master cylinder. It's worth double checking to make sure there are no leaks anywhere else in the system as well.

Good luck,

~Dave
'75 B
Dave Ryzuk

Dave's answer maybe the most prudent. BUT saying that, unless you've serviced anything on the brakes, its probably the MC. If you pump up the pedal and then keep moderate constant pressure on it, does it slowly eventually move? If so, I'd say rebuild/replace MC. I always replace because there is few things as important as your MC. I've had a MC fail and believe me that 79 compression engine will not down shift and compression brake very quickly before you kiss something.

SAFETY FAST, don't screw with a possible brake problem. If you know the MC to be original, I wouldn't even question it beyond the horrid price :D
Luke Staley

leeload: the one rule you must remember is:"Fluid is not compressible while Air is compressible" What does this mean to me ? Air gets into your brake system because of a failure any where in the system, once its there the only way to get the pads and shoes to do their job is to compress it [pump the brakes],as high pressure air will do the job, to correct the problem find the source of the air [wheel cylinders,brake lines, brake hoses,master cylinder and any of the conections] Find the cause fix the problem. Always the safe and cheap way. It may in fact be your MC but be sure. Ric
RIC LLOYD

Don't forget the basics...check the rear brake shoe adjustment. It has to be done manually. The rear brake shoe adjustment is important as it affects the pedal travel and also affects the emergency brake/hand brake operation, and THAT is how you are going to stop your B if your master/hydraulic system goes away. The interplay between the shoe adjustment/hand brake needs to be addressed only the first time really. Once done properly the pedal travel and hand brake travel will always come back with rear shoe adjustment. Do the whole book process the first time just so you know it is right.
Rick

This is great feedback. Thanks! I did recently install a new wheel cylinder and believe I got all the air out (seemed to be working fine for a couple of weeks anyway) but that is the first place I'll start - rebleeding the the lines. I'll also relook for possible air intake and/or leakeage. The emergency brake seems to work fine (nice to know). I don't look forward to MC repair work but that will probably be the last resort. After all, the wheel cylinder needed replacing. I would guess the other rubber components are in need as well.
leeload

Sounds to me you have air in the lines. Bleed the brakes from the furthest out to the closest to the MC. US cars bleed right rear, left rear, right front, left front.

After bleeding and adjusting the rear brakes, depress brake pedal and hold. If it begins falling you need to replace the MC.
Lee

I find conventional bleeding isn't enough on both my cars, I get a long initial pedal that can be pumped up. After basic bleeding I have to get someone to stand on the pedal while I rapidly open and shut each caliper bleed niplle in turn, which always gets another 'lump' of air out, after that it is fine.
Paul Hunt 2

I had the same problem and tried everything to correct it. New MC, new flex lines, and numerous bleeds. Nothing corrected the problem. I suspected a bad new MC and called the supplier in N.C..
He suggested I check the front wheel bearings. They had been replaced just before I purchased the car. Sure enough I jacked up the front of the car and both wheels had too much play. I took the play out of them and the brakes are perfect. Who would have ever thought??
GOT BRAKES!
Roger
R W Cappello

Very cool RW, Paul what do you mean standing on the pedal? Pumping it then holding the pedal down while you open the nipple ? Or tightening the nipple before all the fluid has escaped ?
James

James - get someone to apply a lot of pressure to the pedal while you open and shut each nipple in turn very briefly.
Paul Hunt 2

This thread was discussed between 02/06/2007 and 13/06/2007

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