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MG MGB Technical - brakes binding

I have a '78 roadster. Driving to work after about 5 miles the brakes completely seized up, car would not move. Got a tow home, about an hour later breaks became released and I drove 20 yards to put car in my garage.

I repaced the booster with recon. unit from NAPA (they have them in remade somewere in PA).It was an identical replacement complete with plastic valve. The master cylinder was replaced about 12 months ago with a unit John Twist rebuilt.

Off we go again to work, brakes bind after almost identical distance, this time I drove it home as they didn't feel completely seized.

Checked all brakes, both front wheels were too hot to keep hand and locked solid by this time.Back brakes fine.

Changed caliper on one side it was locked solid , changed rubber flex hose, other side OK.

Bled brakes again, off we go. same result.

dave florida

I'd change both rubber flex hoses...they get old & swell inside, won't let the fluid back to the MC...
Bob Dougherty

Which wheel is locking up? If you loosen the bleeder will the wheel release?

Sounds like somewhere there is a blockage that is not letting fluid return to the master cylinder, so as the system heats up, the fluid expands and applies the brakes. Obviously on the front half of the system. Did this happen before the new master cylinder was installed? I had a Toyota once that had a small piece of grit block a return hole in the M/C, and it caused the exact same symptoms you describe; not sure if this could happen on the MG because said hole on the Toyota M/C was extremely tiny, yet IIRC the hole on the MG M/C is fairly big.
I'd be less suspicious of the flex hose because there is one for each front wheel, and it sounds like both are locking up. Also, you changed one of them?
Erick Vesterback

If both front brakes (sic) are locking-up then it would be a huge co-incidence if both flex hoses had chosen to act as a check valve at the same time.

But as Kimberly says, opening *one* front bleeder will reveal all. If it only releases that brake and the other is only released when opening its bleeder, then both flex hoses are indeed at fault - or two other blockages between the calipers and where their pipes join together.

If opening one bleeder releases *both* front brakes, then the blockage is in the common pipe between the master and where the two front pipes split off.

Having said that the later split system has the individul caliper pipes going right back to different ports on the master, so the master could be at fault in either of the above situations.

Have you adjusted or knocked your brake light switch recently? This relies on the pedal being released to operate the switch which turns the lights off. If the switch is screwed too far into its bracket it will stop the pedal coming back far enough to clear the bypass port and allow heat expansion of fluid to escape into the reservoir. It is a characteristic of the master design that I think this will affect the rear portion of the master first, i.e. it will affect the front brakes first.
Paul Hunt

Some good comments, thanks
Both front wheels locked up.
I changed one front rubber hose, as I do have another hose I will change at the weekend, but why would one bad hose affect both brakes?
Will do more work at weekend and follow up on ideas.
When I recently put in the new servo/booster I naturally had the break peddle hanging loose, could I have upset something then, but wait a minute the whole reason for the new servo/booster was because the brakes locked up in the first place, the master cylinder has been in for about 10 months.
Any other comments before the weekend very welcome
Dave Florida

"why would one bad hose affect both brakes"

Exactly, it can't. Which is why it must be something else, unless by some huge co-incidence both have failed in the same way at the same time. Which is why opening one bleeder at a time will tell what is happening.

I'd be looking at the brake light switch first.
Paul Hunt

Did you find out what was causing the brakes locking?
Would be interested to know.
Trevor Harvey

I checked the break switch light connection that had not moved and I still had the 1/8 gap on the brake peddle.

I emptied the fluid from the MC. I went to the side with new caliper and hose, wheel was off and I could turn the hub but there was friction, not alot but it was quite hard to turn by hand with the wheel off, one pad was a little tighter than the other.
I filled the MC again about 50%.
I put a new rubber hose on the side without the new caliper
I openend the bleeder at the other end ( brakes still solid ) went back to the side with new caliper/hose and forced the pistons out pushing fluid back into the MC and it shot out of the open bleeder.
I don't think there is any blockage.
I am going to get another caliper this week and have the new one ( recondioned one) checked out.

why wont the brakes release ?
Fluid flows OK new rubber hoses both sides.
Should I see some resistance when turning the wheel, ie the brake pads dont completely retreat I always expected some resistance, how much ?
when the foot is off the brake fluid goes back into MC, how/what pulls it back up?

How important is it to get matching brake pads, shouldn't the pressure just compensate ?

Thanks, any help appreciated


If there is no fluid pressure but the pads are binding then the pistons must be binding in the calipers, especially if one pad is being pressed against the disc harder than the other.

Because there is the same pressure on both pads in the caliper, and both calipers for that matter, all pads must have the same friction coefficient or you will have unbalanced braking. This probably doesn't matter much if you have the combination of pads both sides, but will cause pulling to one side if the sum total of friction one side is different to the other.

But if you mean you change *all four* pads for ones with a different friction coefficient, but those four pads are all the same as each other, then yes you will simply compensate with a different pedal pressure for lower or higher rates of retardation than before.

But don't think you will automatically get better brakes by changing the pads or discs. Retardation is controlled primarily by the grip between tyre and road surface - as long as you can lock the wheels that is, which you should always be able to. If you fit pads with a higher friction coefficient, you will simply lock the front with lower pressure. The pressure on the back is the same as the front, so by having less pressure on the front you have less pressure on the back, so *lower* retardation from the rears than before, hence lower retardation overall.
Paul Hunt

My comments might not help your problem, but might help some others. I have an 80LE and haven't started it in over a year due to extensive work. It's a wonderful car, but going through rear suspension rebuild, new interior and lots more details. However, I noted something interesting while cleaning up the back axle during suspension rebuild. Both brake "pipes" have been crushed at approximately the same point when measured in distance from the wheels. They're flattened enough to either totally close or at least significantly reduce brake fluid flow.

It occurs to me that I bought this car on E-Bay (like many of us have and will) and it was transported on an uncovered auto carrier from North Carolina to Missouri. It's evident by "rub marks" that something like a cable over the back axle was used to hold the car down during transport. I can't be sure that the transporter did this, could have happened long ago by a tower or other transporter. The rear brakes have never been very good and the right rear would lock up when I braked more heavily than usual. Those of you that have your cars transported might want to inspect the brake pipes for this sort of damage. A tower or transporter might not be aware that the brake pipes are right on top of the axle housing and easily crushed.
Rick Penland


You said, "I openend the bleeder at the other end ( brakes still solid ) went back to the side with new caliper/hose and forced the pistons out pushing fluid back into the MC and it shot out of the open bleeder.
I don't think there is any blockage."

This statement is a little confusing to me, but it sounds like you shoved the pistons back into one caliper, fluid shot out of the open bleeder on the opposite side, and you concluded that brake fluid was returning to the master cylinder.

If I understand what you did, I don't see how you came to the conclusion that it means that fluid returned to the master. It can simply go from one caliper to the other without returning to the master.

If you shoved the pistons back into the caliper with all of the bleeds closed, then the level in the master should increase and show that it is returning. I think there is a small return hole in the master that is prone to plugging, or if the master piston is not returning all of the way home, the piston and seal can block the fluid return port.

You could have the pads sticking in the caliper, and if so you should clean them up and get them moving freely, but it is not that likely that they would jam up so tight to require a tow home. If you can open the bleeder on a wheel, and that brake still drags a lot, you may have this problem, or the pistons may be stiff in the bore.

If you pull the pads out, how hard is it to push the pistons back into the bore? They should not be too hard to move, though it may be a bit more than you can easily do with no tool to lever them.

As far as mismatched pads are concerned, that could cause unbalanced braking, but should not cause the problem you have.

C R Huff

Thanks for response on Dec 17th, I have been away in Ottawa for Christmas visiting, just got back.
I have a '78 B and with individual lines from the MC to each front brake wouldn't this mean the fluid from the one brake would be pushed back into the MC and then to the other front brake !
With regard to the calipers I think this may be the problem. I am going to take them both off and take them back to the shop I got them from for testing. I have new rubber hoses on both front brakes now and the booster replaced ( and the MC about 12 months ago).

Sorry, one last question for now, how would I best check if the MC piston is not returning all the way home ?

Think I would go with the previous comments on the brake light switch. Just loosen the nut and back it out a couple of turns and try it. I had the exact same problem and the switch was faulty. It doesn't have to hold the brake pedal down far, just enough do do what you're describing. This is the easiest of all the possible solutions I've seen offered here.
tom (member)

"wouldn't this mean the fluid from the one brake would be pushed back into the MC and then to the other front brake"

*Pressure* from either caliper is released into the reservoir, not into the other caliper, unless possibly the feed port from the reservoir was blocked. This would be the case if the piston wasn't coming back far enough to clear the port (like when a brake light switch is screwed in too far...).

Because the section that feeds the front brakes is closer to the pedal than the section that feeds the rears if the front section piston wasn't coming back then neither would the rears, and all four wheels would be locked up. In that case slackening one caliper nipple would release the pressure from both calipers but not from the rears. You would also not be able to get any fluid from the reservoir to either caliper (or rear slaves for that matter) by pressure or suction bleeding i.e. with a gunsons EeziBleed or Mityvac.
Paul Hunt


Yes, with the separate lines, I think the fluid would be going back to the master before it goes to the other caliper. However, I think the blockage could be where the fluid from the bore in the MC has to return to the reservoir (instead of between the caliper and MC).

If it used to work fine, and you have changed nothing to do with the MC, then extra free play in the pedal may indicate that the MC piston is not returning all the way. That is, if you pull up on the brake pedal, you can then feel how far you have to move it before the push rod contacts the piston. If that distance is significant, it may indicate that the piston is not returning all the way, and thus not letting the pressure from the bore return to the reservoir.

I’m not exactly sure what you meant when you said you had an 1/8 gap at the brake light switch. If the pedal wasn’t touching the switch, wouldn’t that mean the brake lights would be on? Also, too much gap there might mean the piston/pedal wasn’t returning all of the way? Anyway, if you want to eliminate it as a problem maybe you could just remove the switch.

C R Huff

Well, this is what I did today.
Got the calipers checked out seals on one caliper needed changing hence the previous fluid leak, this should have been done at the shop, they left the job with the helper not the tech guy, I have a free caliper for my troubles, still would recommend the shop.
Calipers on, fluid flows to calpiers, got a good firm brake peddle, brakes locked on and would not release let fluid out of each nipple brakes released but still had contact with rotors.
Took off the MC. Nothing obvious to see
I pulled at the piston sticking out of the booster and came loose and I was able to push it back in place without it coming off. can't remember what it was like when I put it in, also when I had the piston hanging loose I noticed it has a threaded section so it could be adjusted.There is a washer on the piston that is in good shape, this will go against the MC cap thing.
In summary:I have renewed the booster,the rubber hoses and the calipers this last month and still have the same problem. The MC was replaced about 12 months ago, maybe 1000 miles ago.
I think I want to clean the MC or check for blockages, how can I do this?
Also, the breaklight switch has not moved in years so why would that come into play?
Your comments really required now.


Had a similarly intractable problem with a 78 roadster with dual circuits where both fronts locked when hot after a winter of low use. Refit for both with new cylinders appeared to sort it but it returned very quickly to the front right which then could be relied on to glow red when least expected.

The long story (not dissimilar to Dave's) led to replacement with a new f.r. caliper.

My conclusion at the time was that at a certain condition of wear/temperature (mine were VERY high mileage) the piston can stick on steps created by wear on the wall of the cylinder (irrespective of the state of the moving cylinder).

I have very little faith in other people's refits unless they have the equipment to put the gear to the most exacting test. This applies to brakes, steering racks, king pins.


Being in contact with the discs, especially when the brakes have been operated and released with the car not moving, is perfectly normal. The amount of grip reduces as they back off a bit, but they won't pull clear of the discs like that. It takes the small amount of end-float in the bearings that effectively gives the discs variable 'run out' when the wheels are turning and the car cornering to do that. Telling us they are still in contact doesn't help, are they still gripping the disc so it can't be turned? And is that the case before or after the bleed screws have been slackened or both? And does slackening one bleed nipple only free one wheel (if it frees it at all) or both?

I can't understand why you are so reluctant to investigate the brake (sp) light switch. It has been mentioned several times that this will cause your symptoms. Saying it hasn't moved (what does that mean? It doesn't work?) for years is ignoring the fact that the switch itself may have gone faulty and the plunger isn't going into the switch body as far as it used to. That will stop the brake pedal coming back as far as it should, fluid will be retained within the calipers, expand as it heats up, applying the brakes independantly of the pedal.
Paul Hunt

OK, thanks Paul, you have me I'm onto the (sp)light switch this weekend.
I expected the brake pads to be in touch with the rotors after I released fluid from the bleed nipples, I was really only trying to describe that this happened as it should.
Each brake released when I opened the nipple for the specific brake, in other words when when I opened the nipple on one brake it released that brake and when I went to the otherside this side was still locked on until I opened the nipple.
What did you think about my comment about the piston coming from the booster being screwed, which makes me think its adjustable, does this make sence?


You said John Twist's shop did the MC. I would not brush over this as NOT being the issue. I had a similar problem with my B. I have a MKI so it is a totally different braking system, but problem was pretty similar. Would drive a short distance and brakes would lock. Turned out that the MC piston was not moving forward enought to allow the fluid to get back to the MC. In my case, the brake light switch (mounted on the MC cover on my car) was just slightly out of adjustment

I would give John Twist a call. He can probably diagnose the issue fairly quickly.

If each wheel only released when its *own* bleed nipple was slackened, that says to me there is some other blockage between the master and the calipers.

If the master were not fully released, i.e. by the brake light switch not allowing the pedal to come fully back or some other cause of the same thing, then I'd expect *either* caliper nipple to release both calipers, for as I understand it they should effectively be connected together via the space in front of the master piston even though they have separate ports on the MGB master. If I'm right then the brake light switch, or indeed anything in the master, can't be the cause (but see the end). The most common cause of this otherwise would be the flex hoses delaminating and acting as a check-valve. If both hoses have now been replaced then the chances of *both* old ones and *both* new ones having the same fault is pretty infinitisimal.

As opening each nipple only releases its own caliper, as seems to be the case, then working back from the calipers I'd start slackening each end of the hoses and each end of the pipes, one at a time, when they are locked up, and see if there is any change i.e. slackening one part releases both together or neither. If you find a point where slackening a joint *does not* release its caliper, then the fault must lie between there and the previous point (towards the caliper), that did.

If you get all the way back to the master and slackening the pipes there still does the same as slackening the nipples i.e. each pipe only releases its own caliper, then the problem must lie in the master. But as to what it could be at the moment I can't imagine, except for the following: If the master piston is sticking so far forwards that it is covering the *outlet* ports, instead of just the bypass port, then it would isolate the two separate caliper ports from each other as well as retain pressure in them. But for that to happen the piston would have to be much further forwards than it should be, and from what I've seen it should get anywhere near that.
Paul Hunt

This thread was discussed between 07/12/2008 and 02/01/2009

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