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MG MGB Technical - New Brakes Lockup

Ok, gentlemen, here is the problem:

My 71 'B with non-servo tandem system is experiencing lockup on all four wheels after driving less than a mile - hardly time to generate any heat. In fact, the brakes will lock and not release while parked in the garage on jack stands - pump the pedal 6-7 times.

I have recently replaced the front calipers w/rebuilt units, had the rear calipers (280z units) rebuilt, replaced all flexible lines with steel-braided items, and installed a rebuilt M/C. I have bled the system to death (wife got bored, bought a mighty-vac unit). I have removed the brake light switch. There are no leaks. I cannot find any air in the system.

So...what is causing the lockup???


Martin Connolly

Hmmn. Possibly a problem with the proportion valve??
MK Mike

Proportioning valve? There wasn't one on any MGB from the factory, brake balance is done by selecting the appropriate size for the rear slave cylinders.

What is locking? All four, just one end, or just one caliper? If all four then it sounds like the brake pedal isn't letting the master pistons come back far enough to clear the bypass hole. The brake light switch is the most likely culprit for that, but removal noted. Also if that were the case you wouldn't be able to get any fluid from the reservoir to the wheels with a continuous device such as a mityvac or an EeziBleed.

The question is, when they are locked what happens when you open a bleed screw? Is there a spurt of fluid and then that wheel is free? If so it is fluid pressure, if not it is sticking pistons. That's the first diagnosis that needs to be done.
Paul Hunt

"...and installed a rebuilt M/C."

Sure sounds like a master cylinder problem to me.

Wayne Pearson

I'm not sure about the 71, but my 73 locked-up as you described with mal-adjustment of the brake light switch. It's not allowing the release. Dennis
Dennis Hosier

Martin. Since you are running a modified system (280Z calipers on the rear), it would be interesting to know what other modifications might have been made to the brake system and which of the original parts are still in use. That would allow people to address what might be wrong with the parts we are familiar with.

With a hybrid system, such as you are using, I suspect that the fastest method of finding out what is not working would be to have the car towed into a good mechanic's shop--one familiar with the system in use if there is such a person in your area.

As to general comments: "In fact, the brakes will lock and not release while parked in the garage on jack stands - pump the pedal 6-7 times." This, if I am reading it correctly, states that if you pump the pedal six or seven times, the brakes are locking up. This would indicate that heating is not a part of the problem and the problem happens when brake fluid is forced into the calipers by the action of depressing the pedal. With that definition of the problem, the question becomes:

What is not allowing the calipers/lines/master cylinder to release the pressure being applied to the calipers?

The most common sources of this inability to release the pressure is damaged rubber hoses (internal collapse) and insufficient travel of the master cylinder pistol to uncover the return fluid holes in the main cylinder bore. Those are the areas I would inspect first.

Les Bengtson


Thanks for the suggestions. To answer some questions:

1. All four brakes lock simultaneously, even though it is a tandem system.

2. The flexible hoses are braided steel, and have been renewed.

3. An adjustable proportioning balance valve was installed when the rear discs were put on. I have not had any trouble with this adj. unit until now. Could a malfunction have occurred just at the same time as I renewed other brake components??

4. When the brakes lock, the only cure is to open one of the bleed screws, whereupon a spurt of fluid escapes, and all four brakes are released.


Martin Connolly

You didn't over-fill the brake m/c reservoir, did you?

And is the vent in the cap unplugged?
Rick Ingram

I would tend to agree with Dennis. It sounds like the piston in the MC is not getting far enough back to relaese the pressure when you take your foot off of the pedal. Free-play is usually controlled by turning the brake-light switch in or out.
David "I also agree with Wayne that it could be a bad MC, too" Lieb
David Lieb

Ok, guys.

I have found that although I had partially unscrewed the brake light switch, it was still causing the pedal to bind. Loosened the switch a few a turns, and suddenly the brakes are brilliant!

"All else being equal, the simplest solution is usually the best...."


Martin Connolly

Glad that you found the fix.

FYI: The brake light switch should be adjusted so that there is
about 1/8" of free travel at the brake pedal.
Daniel Wong

"1/8" of free travel at the brake pedal"

I've often wondered just what that measurement means. Logically it should be '1/8" further travel of the pedal (pad) after the master push-rod has stopped moving', i.e. you need some play in the pedal to push-rpod linkage. But that is impossible to check with the brake light switches on the pedal box at least, and I'm not even sure how you would do it on the tandem system with booster either. Probably easier to unscrew the switch till the lights come on, screw it back in till they just go off, then one or two further turns. A neighbour has the opposite problem - his lights on his Audi tend to stick on. In cold weather they will come on over night all by themselves.
Paul Hunt

Just read the thread. Glad you fix your problem. Do you mind shedding any light on your 280z rear caliper conversion ?

This thread was discussed between 21/04/2009 and 23/04/2009

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