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MG MGB Technical - Brakes binding when hot

A friend at my MG CC has a problem he can't solve. I am a midget owner so I thought I'd ask people on this BB who might know better than me what the problem could . His car runs OK most of the time and the brakes work perfectly. It recently passed the MOT with 'flying colours' (brake test and all). Then, without warning (but usually when hot after a run) the front disc brakes lock on. This has happened off and on for 12 months or so and he can't trace the fault. He reckons the rear brakes are not affected. He has changed all of the flexible hoses so that is unlikely to be the cause (no collapsing hoses etc). He even changed the brake pads in the hope this might have something to do with the problem (no effect). I suggested looking at the piston seals (gone stiff and not retracting the pistons?)or the master cylinder. Quite reasonably he is reluctant to start changing items just to see if it makes any difference (it would soon get expensive). Any suggestions, oh MGB gurus?

Chris Hasluck (1970 Midget 1275)

Sounds to me like the master cylinder rod is too long and not allowing the master cylinder piston to come far enough back to clear the vent hole. Thus as the brakes warm up the pressure is trapped in the system and brings on the brakes. Did someone adjust the braking circuit for MOT?
Bob (robert) I am turning? yes I once owned an MGWasp!!

Hi Chris.

With no pressure on the pedal there shouldn't be any way for pressure to build up in the hydraulics unless...

It's a long shot, but maybe a blocked master cylinder cap vent hole would allow the build up of pressure when the fluid gets hot ?.

BTW, Does the car have a servo ?.

D P Malin

Don, Bob - thanks. My thoughts are along these lines. I have suggested the master cylinder as the cause but some people are reluctant to take advice (however nicely it is put) so that was one reason I sought some advice here (he might believe you!).
I believe it has a servo. That could be disconnected to see it it is the cause (perforated diaphram?).
Chris Hasluck (1970 Midget 1275)

Hi Chris
I once had the same problem on a Toyota Celica - front discs would lock after some time driving, but the rear drums would stay free. The hole for the fluid to go from the reservoir into the master cylinder was so small that even tiny pieces of dirt in the system would lodge in there, and cause the system to pressurize as the fluid heats up. I suspect the rears never locked up because, being drums, they had some slack that would have had to be over come before they got to the point of locking.
I don't know how small the holes in the master cylinder of the midget are (My MGB holes are way bigger), but I suspect something is wrong in that area, maybe just as Bob has indicated. Replacing the master cylinder in my Toyota solved my problems. I really doubt it is a caliper problem, especially if both wheels bind.

Hope it helps.

Erick Vesterback

Is he certain that the rear brakes aren't affected? If they are and if he has a servo, then consider that the tiny piston in the servo can sometimes become sticky resulting in the brakes staying on. It's easy to remove and clean it, and often it isn't even necessary to bleed the brakes afterwards.
Miles Banister

What year? If it has the mechanical brake light switch screwed into the pedal box then that is a classic symptom of the switch being screwed in too far. Haynes quotes 1/8" free play at the pedal footpad, although that is for the tandem master cylinder, there is no information for the single circuit system. You could try screwing out the switch until the brake lights fail to come on, then screw it in counting the turns until it obviously removes all the play from the pedal, then back it out half the number of turns.

Remote servos also suffer from sticking on in hot weather. The other symptom is that when stuck on the pedal barely moves at all before applying the brakes still harder, but tapping the pedal tends to release them. I've seen two fixes for this - one is to carefully remove the piston from underneath the air-filter on the remote servo and lubricate it with silicone grease. The other is to rotate the servo so the air-filter is pointing downwards and not up, presumably to keep the piston lubricated with brake fluid. I've tried the first method recently (too early to tell) but I noticed the piston was already mostly coated in brake fluid. I gave it some more higher up, but don't hold out much hope.

Both these affect rear brakes as well on single circuit cars, and the brakes lights where there is a hydraulic switch.

Blocked hole in the breather cap will also affect backs and fronts, single and dual circuit systems, and is easy to check - just loosen the cap when they are locked on.

The only other thing I can think of at the moment is *both* caliper hoses delaminating inside and acting as one-way valves, but both happening and not happening at the same time would be quite a big coincidence.
Paul Hunt 2

im having the same problem with all brakes locking on
we think it is the servo seal but my dad thinks it is the brake fluid which is fully synthetic which doesnt lubricate as well as normal fluid so seals tend to stick in hot weather.
are any of you using fully synthetic who are having this problem?
Ste Brown

For my part it is non-synthetic.
Paul Hunt 2

PS. V8 with the identical system and fluid does not experience the problem.
Paul Hunt 2

I have the same problem with a 1971 MGB and a 1970 E-Type Jag. This is a new problem, occurring for the first time with both cars in the last month. With both cars, it happened on a very hot day, temps in the mid 90's (F), and in almost gridlock traffic. Bumper-to-bumper, stop and go, with more stop than go. I am not sure about the MGB; but on the Jag, I think it was front only. By servo, I assume you mean a power brake unit. The Jag has one, the MGB does not.

I live about 15 miles out of Gatlinburg where many of you experienced similar traffic at MG 2006. My cars are routinely subjected to these conditions and I have never had the brakes overheat and lock up before.

Ste Brown raises a question that has been bothering me for some time; and I realize that I am about to commit hearsey here. Other than the way they react to natural rubber seals, is there a difference between UK spec brake fluid and US spec brake fluid and could a failure to change the UK spec fluid cause this problem? The recommendation on this BBS and in the manuals is to change the fluid periodically. At various times in my life, I have owned the same US vehicle for more than ten years and more than 150,000 miles. I have never had a master cylinder failure or wheel cylinder failure and I have never had to change a flexible hose. I have also never changed brake fluid. The MGB brakes were completely overhauled about five years ago with new master cylinder, new calipers, new wheel cylinders, etc., and new fluid. The fluid hasn't been changed since. I have no idea how old the Jag fluid is. I have had the car for at least five years and the fluid has not been changed during my ownership.
A Sneed

A Sneed; Your Jag and MGB probably spend lots of time sitting and are not driven daily. DOT 3 or DOT 4 brake fluids will absorb moisture over a period of time. The moisture reduces the boiling temperature of the brake fluid. In the hot weather, in stop and go traffic the brake fluid temperature exceeds the boiling point, the moisture loaded boiling fluid may be causing your brakes to lock up. My theory may be all wet, no pun intended. I would replace the brake fluid in both cars.

I have noticed that DOT 4 fluid in my MGB will start turning yellow in a few months and in about two years the fluid starts going brown, I change it at that point. My daily driven cars come with DOT 3 and it takes much longer for the fluid to change color. Daily driving seems to help reduce moisture absorption. BTW, I change the brake fluid in my daily drivers every 2-3 years.

Clifton Gordon

Thanks for all the suggestions. I will pass them on.

Chris Hasluck (1970 Midget 1275)

I've *never* changed brake fluid in 40 years, in cars I've had variously up to 15 years or 70k miles. And yes, I know it is supposed to be changed routinely. I have changed brake hoses on the '15 year' car, the caliper hoses for the 2nd time just this week, but only because of external surface cracking which is an MOT (annual test) failure here in the UK. My understanding was that boiling of the fluid was more likely to be noticeable as a spongy or bottoming pedal rather than the brakes locking on. If with the brakes locking on the pedal is as hard as normal, or harder than normal as in my case, then I doubt it is boiling fluid.
Paul Hunt 2

This thread was discussed between 03/07/2006 and 07/07/2006

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