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MG MGB Technical - MGB Brakes problem

i have a 1970 mgb gt. the brakes are not working. on bleeding the brakes, the brake fluid is coming to all 4 wheels, but the brake pedal is not holding pressure and sinks right down. any ideas?

zak
zak

Sounds like the master cylinder is shot. Usual advice is to get a replacement rather than try a rebuild, although some have been successful.
Steve Postins

"Sinks right down but there is fluid coming out of all 4 wheels".

Zak Clarify a bit. If you open the bleeder and push the brake pedal does fluid squirt out?

Does the peday go easily to the floor or do you get some resistance and then it fades to the floor.

Have you added fluid lately and if so, what type. I would pull the master and take a look at it. Rebuild rubber kits are inexpensive. If you have the time, may be worth trying.

Bruce Cunha

The description fits a shot M.C.If you can get hold of Lindsay Porters Guide to M.G.B. theres a good overhaul of the M.C.section in there
Bob
R Etches

My 67 GT has on 2 occasions about 45 days and a few hundred miles apart had the left front disc brake start dragging for no apparent reason. After letting it stand for a 1/2 hour or so, the brakes seem to operate normally.

I wonder if something is floating in the wheel cylinder and periodically floating up and blocking the release of the brake fluid from the wheel piston/cylinder.


Barry
Barry Parkinson

Let's get back on track. Yes it could be the M/C that's shot but it could equally be incorrect bleeding. I've put bleeding procedures on this board a couple of times and until we are sure that they are bled correctly I'd be reluctant to trash the M/C.

Bleed the system again Zac and preferably with something like an Eezi Bleed this time.
Iain MacKintosh

Zak - if the pedal really does sink all the way to the floor while you are applying pressure, and there are no leaks, then the seals in the master are bad. But even with good hydraulics I find that simply using an EeziBleed or the pedal to push fluid through the system is not enough and I get a long initial pedal, that can be pumped up (for a while) with repeated strokes. To cure that I have to get someone to stand on the pedal while I rapidly open and shut each caliper nipple, and that always blasts a bit more air out, and after that the pedal is normal.

Barry - sounds like classic front brake hose degradation. Flaps of rubber delevelop inside and act as a check-valve - letting fluid in but not back out, so maintaining pressure on the pads. The way to check is to slacken the caliper nipple imemdiately after standing on the brake and while the caliper is still locked. If some fluid spurts out, and the disc is then free, then it *is* the hose (or possibly debris blocking the exit hole but this is unlikely). However if fluid only dribbles out and the disc is still clamped it is more a case of siezed caliper pistons.
Paul Hunt 2

Barry

Paul is probably correct. One item to check. Has your 67 had the brake converted so the brake switch is on the pedal cover? If so, you maw want to back it off slightly. If it is putting any pressure on the pedal, the piston in the cylinder may not be clearing the return hole completely.

I had a dickens of a time figuring this out. Replaced all hoses, new calipers and I still had a sticking brake.

Then I found a two sentence piece by Jon Twist talking about the brake light switch. That was the problem all along.
Bruce Cunha

I've seen Paul Hunt write this procedure before and cannot understand why it should be necessary. Nevertheless it seems to work for him. Just a thought though - the bleed nipples should be installed in the uppermost port in each calliper and the feed fipe to the lower one. If this is reversed I can then understand why a pocket of air is becoming trapped. Check yours Zac to ensure that you are correct.
Iain MacKintosh

Both cars, Iain.
Paul Hunt 2

Strange one, sometimes it can be difficult th bleed the last of the air from the servo as it tends to become trapped under the reaction piston but you seem to get trapped air in each caliper. I wonder also if Zac is using silicon fluid as this is very much easier to aerate than with standard mineral types. Looks like you don't use silicon as you method would quickly froth it up !!
Iain MacKintosh

Berry,

I had a similar situation after I replaced the master cylinder on my 71 GT. The brakes would start to lockup after less then a mile. I had to bleed the rear slave to release the pressure on the system so I could nurse the car home. Turned out that I had neglected to replace the springs that bull the peddle back after your depress them. These springs attach to the peddle body about 2/3’s up and to the under side of the firewall. Without these the peddle will not retract completely so the fluid can not return and relive the pressure applied to the pads.

Hope this helps.
Ron
R Garcia

Another cause of Ron's problem is a poorly adjusted (mechanically operated) brake light switch. If screwed in too far it will stop the pedal coming back all the way. Switch should be screwed in about mid-way between the lights just going of when the pedal is released, and just starting to obstruct the pedal. You can have it so that it *is* just obstructing the pedal but does not caus the brakes to stick on, but sliding your foot sideways off the pedal to release it suddenly will mechanically shock the switch and shorten its life.
Paul Hunt 2

This thread was discussed between 24/10/2005 and 03/11/2005

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This thread is from the archive. The Live MG MGB Technical BBS is active now.