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MG MGB Technical - Brake problem

A friend of mine owns a 1975 US-spec MGB, with brake servo and has the following problem:
when driving, at first there is NO problem
BUT after having driven a few miles, the 'play' of the brake pedal is gone and later on, the front brakes drag, even without actually pushing the brake pedal. When this occurs, the brake lights keep working.
After letting the car cool down for a while (half an hour), the brakes act as they should (little bit of play, brake lights working only when brakes are applied).
When the servo is disconnected (by disconnecting the vacuum), the problem seems not to exist, so it probably will be the servo.

Does anyone have a clue how to solve this?

Thanks in advance,

Alexander
Alexander M

Sounds like the brake fluid is being overheated by either binding/dragging brake pads or shoes, or most often insufficient pedal free play. In your case it does sound as if the piston in the master cylinder is not returning to the rest position, having the same effect as not enough free play or "riding the brakes" with your foot.
If the fluid is too hot it expands, is less compressible, and you lose all pedal feel, i.e. the pedal becomes rock hard. The expansion also explains why the brake light is on all the time in this condition.
Andrew Blackley

A valve in the servo opens under pressure from the master cylinder when the brake pedal is pressed. This valve allows engine vacuum to be applied to the large diaphragm in the servo which applies increased pressure to the brake system. When the pedal is released the vacuum is allowed to bleed off and the system returns to neutral. If that servo valve is leaking or stuck in the open position it will cause vacuum to build on the diaphragm and pressure to build in the system, and the symptoms you describe will occur.
I don't know if the servo can be (easily) repaired or not.

Bill Boorse

I had exactly the same symtoms with mine, although mine is a 73GT with remote servo.

I eventually sorted it only last week, in short it turned out my master cyclinder had a whole lot of gunk in the bottom stoping the piston from the brake pedal returning fully. I believe yours will have the tandem/in line servo so might not be the same thing.

In the process of finding the problem I have replaced the calipers and cleaned the servo pin both of which relieved the problem enought to drive for a few weeks then it was back.

When the master cylinder and piston was taken apart and clean up and refitted everything was fine again.

The master cylinder must have been repeatedly topped up rather than drained and refilled for many years by DPO allowing this nastiness to gather in the bottom.

I am assuming (probably to be correct as I am bit new to this myself) with this nastiness in the bottom the main piston in master cylinder would have had major problems returning to rest position. The resulting pressure of the piston not being at rest would have caused pressure to stay on the brake fluid, this kept the pin of the servo in engage position.

Obviously the servo being in the engaged position would have dutifully increased the pressure on the fluid - thus causing the brakes to gradually engage over the period of a few mins. As the pressure built up the pads and disks would have made increasing contact eventually causing full brake pressure to be applied thus stopping the vehicle.

When the engine stops the servo eventually loses vacumn and the brakes are released (with a groan on mine) when the engine starts again the servo is still engaged and starts building up pressure again and the cycle continues.

Like I say I only got this far with help from this group so if any of the above is argued with you are probably best listening to them
Simon Crozier

Alexander
A few years I had what seems to be similar symptoms to those you describe. (Brakes binding on for no reason). I changed the remote servo and this cleared the fault.
Cecil Kimber

May I add to the conversation?
Thank you.
Well my sad story is, I changed from the stock power booster to the earlier , out of a '72 GT.
Now my brakepedal, even with the braided flex hoses is spongy.
Ihave bled and re-bled. I have pumped backwards from the calipers and rear wheel cylinders. Still a bit of a low pedal and spongy. It's been suggested that there is a spot that is higher that retains air. I am open for any suggestions.
Thanks
Safety [slow me down] Fast
Dwight
Dwight McCullough

I had this problem on a 74 mgb. The question is, is it only the front brakes that are locking up? After much trouble shooting and many attempts to fix, it turned out to be the proportioning valve on the brake lines. This little valve is easily over looked and rarely will it fail. but when it does, it will trap the brake fluid on the front brakes. If you let it sit for some time it corrects itself. The fix? Take it apart, blow some air through it which will dislodge the little piston inside, clean and reassemble. This worked for me and the problem has never returned!
Hope this helps.
jtm maguire

The mechanical brake light switch can be screwed in too far which stops the pedal and hence MC piston fully returning, and subsequent heat can cause the fluid to expand and apply the brakes. Normally any expansion of the fluid simply goes back into the reservoir. The remote servo also has a failure mechanism in that it can lock-on, but this can usually be released (unless very bad, at which point you disconnect and plug the vacuum hose from the inlet manifold) by tapping the brake pedal.
Paul Hunt

Thanks for you input everybody!

I must however clear a few misunderstandings first I'm affraid:
- The odd thing is that this problem does NOT exist when the servo isn't used, so problems with the bleeding or the brake fluid itself seem not to be the case. (or am I wrong here?)
- It's probably not the brake switch too, because when my pal screws it out (totally that is), the problem still exists
- The front brakes don't really block. My pal thinks that all brakes start to work lightly, but this can only be heard from the front disc brakes when cornering
- The servo I mentioned is NOT a remote servo, it is the other type (mechanical, if that is the correct term).

Please keep adding possible solutions!

Thanks in advance!!!

Alexander
Alexander M

If the problem doesn't occur when you are not using the servo (vacuum pipe disconnected?) then the problem would appear to be in the servo, which is what you first suspected. Haynes doesn't go into servo repair, my Leyland Workshop manual does, but it doesn't cover the later integral servo, only the remote. Service kits are available for the remote type, at least.
Paul Hunt

The servo not working is what I said it was!
Cecil Kimber

Cecil is right! Same problemo with me on my 71 retrofit servo. New master and replaced servo with good working used and voila. Vic
vem myers

This thread was discussed between 17/07/2003 and 19/07/2003

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