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MG MGB Technical - Brakes hanging on

Hi all
The front brakes on my '69 B tend to hang on a little after I've stopped, and take a couple of seconds to release. Would any of you clever folks know what causes this please? May I also apologise for always asking questions but rarely answering other people's queries - this is simply because I know absolutely nothing!
Cheers, Rich
r j symons

Have you checked the flexi hoses, they have a tendancy to act as a one way valve.Do you have some free travel on the M/C pushrod.The free travel is important as the recuperation port needs to be uncovered when the brakes are released, so that there is no residual pressure in the system.

regards John
john wright

Does your car have a servo fitted?
Dave O'Neill 2

Thanks for replies, guys. John, I will check the master cylinder pushrod for that port, but as far as the hoses are concerned I guess that replacement is the only option. Dave, yes it does have a servo. Regards to you both, Rich
r j symons

disconnect the servo hose from inlet manifold and plug the hole and see if problem goes away. if it does then servo no good. just had same problem on my mgc. servo was the problem
regards bob.
Bob Taylor

You may also try adjusting your brake light switch located on the pedal box above your master cylinder. Turn it clockwise one turn and see if this eliminates the problem. RAY


UK cars still had the hydraulic switch until mid '70s.
Dave O'Neill 2

Ok, will try the servo hose too. Yes, my brake light switch is hydraulic - bloody thing only works if I brake reasonably firmly. A friend once mentioned something about changing the caliper seals
r j symons

my brake lights only worked when braking hard too. servo changed and all good.
Bob Taylor

My roadster servo also tends to cause the brakes to stick on, although only in very warm weather. Tapping the pedal usually clears it, although on a pals when touring it was so bad we disconnected and plugged (with the pointed end of a spare spark plug) the vacuum hose at the servo end. Incidentally doing that made no difference to braking effort that I could detect, nor on a car that I didn't even realise didn't have a servo. That's the remote servo on single-circuit brakes, the later integral servo on the dual-circuit master is said to give a lot more assistance and probably not safe to drive without the servo.

Fixes are said to be a) lubricating the air valve with brake fluid (which made no difference on mine), b) honing the air-valve piston bore, or c) inverting the servo so that the air valve assembly is pointing downwards.

A brake hose will only cause one calliper to stick, the servo both, and the rear brakes of course, although with the fronts binding you aren't really aware that the rears are as well.

It won't be the calliper seals, they wouldn't both come and go together, and with only one it would pull to one side, and is unlikely to do that so hard you would notice it while driving. Callipers do stick, but usually the first you know about it is a burning smell and blue discs, and maybe a sense that it isn't pulling quite as well as normal.

tried all them things you say paul and did not work on my C servo.had a bgt that i took servo off of and couldn't tell the difference when i drove it but the C is a totally no go without the servo, you have to stand on the pedal.
Bob Taylor

tried all them things you say paul and did not work on my C servo.had a bgt that i took servo off of and couldn't tell the difference when i drove it but the C is a totally no go without the servo.
Bob Taylor

As John said earlier could be the front flexy hoses, They can degrade and block and act as a one way valve. The only way to check is to replace. I have no servo on my roadster but I have totally rebuilt them and they are very good, certainly do not require a servo.
Trevor Harvey

The brakes on a servo equipped car are identical to those on the earlier non-servo car, so it obviously doesn't need the servo. It was simply a change made because as the years went by drivers expected a lighter press on the pedal. In my experience of MGBs, Midgets and Lotus Elans, none of them need a servo. In fact my opinion is that the brakes are more reassuring without one.
Mike Howlett

I'll agree with that Mike
I like a good firm pedal
As a matter of fact, years ago on a competition B I had I increased the brake master cylinder to a one inch one to stop the brakes locking under angry braking
You could really stomp on the pedal without the fear of it all going pear shaped under brakes

You could try the servo hose off trick, but first off ,just to eliminate possibilities - If your hoses are more than a few years old I'd replace them first including the one from the body to the rear axle
At least then you will know that they are all good
If that fixes it - good
If not continue on with your diagnosis but with nice new hoses---can't go wrong--

William Revit

As far as I know, MGB brake master cylinders have a residual pressure valve designed to hold about 6 psi in the system.

This is to keep the brakes shoes/pads close to the drums/discs.

If your brakes are holding on for a couple of seconds, I wouldn't worry - it sounds about right to me.
Chris at Octarine Services

I can remember years ago when PBR were selling VH44 booster kits for the old Holdens here-- They had a similar pressure retention valve in the end of the brake M/cylinder and it had to be removed when fitting the booster kit or the brakes would hold on big time--
But that's not an MG---just something I remembered and just had to tell someone--
William Revit

Residual pressure valve:

I've heard of Americans talk about this on their early dual-circuit unboosted systems, but it doesn't apply to the single-circuit master on the MGB.

There IS a valve in the outlet of the single-circuit master, but rather than holding a few pounds in the system, it is a one-way restriction.

This valve contains a spring covering a large port (arrowed on the left) that opens fully when you apply the brakes, but when you release the pedal the spring releases to close that port, leaving just a very narrow channel (arrowed on the right), which restricts how quickly fluid can flow back into the master.

It's this feature that allows you to 'pump up' the brakes when there is air in the system, or the rear shoes are badly out of adjustment, but it does not result in pressure remaining in the system.

As said earlier the flex hoses can degrade and maintain pressure in a caliper, but this would only affect one side at time especially as it is coming and going, and if it was bad enough for you to be aware of it would cause pulling to one side.


Chris, the Ford Transit had one of those valves fitted when they changed to disc brakes.It was mounted on the bulkhead near the M/C.I have a feeling that the Transit RPV was set at approx 10 psi, was for as you said to keep the pads close to the discs. Maybe it was fitted as Ford were not good at making discs/ hubs that had too much runout.
Wish I had 1 for every Ford drum and disc I machined to true them up, I would be a wealthy man.
john wright

Ford Transit---------bbbrrrrr--- a shivver, I just felt
William Revit

The MGA master cylinder has a slow return valve in the system. See the MGAguru's explanation here
Hope this helps.
Mike Ellsmore

Same as the MGB.

well do we know the outcome yet.?
bob taylor

Come on Rich give us the answer..............assuming you have one.
Just one caliper.......................flexi, both...............sticking air valve piston on the servo slave.
Allan Reeling

This thread was discussed between 17/03/2017 and 03/04/2017

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