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MGB Technical
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Thread: Brakes hanging on
Posted 17 March 2017 at 14:48:45 UK time
r j symons, Nottinghamshire, United Kingdom

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

Posted 17 March 2017 at 14:58:29 UK time
john wright, West Yorkshire, United Kingdom

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

Posted 17 March 2017 at 15:29:09 UK time
Dave O'Neill 2, West Midlands, United Kingdom

Does your car have a servo fitted?

Posted 17 March 2017 at 18:50:20 UK time
r j symons, Nottinghamshire, United Kingdom

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

Posted 17 March 2017 at 20:54:21 UK time
Bob Taylor, Essex, United Kingdom

rich,
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.

Posted 17 March 2017 at 21:22:49 UK time
small, threadlist, tables1

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

Posted 17 March 2017 at 21:49:15 UK time
Dave O'Neill 2, West Midlands, United Kingdom

Ray

UK cars still had the hydraulic switch until mid '70s.

Posted 18 March 2017 at 11:39:10 UK time
r j symons, Nottinghamshire, United Kingdom

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

Posted 18 March 2017 at 13:09:24 UK time
Bob Taylor, Essex, United Kingdom

rich,
my brake lights only worked when braking hard too. servo changed and all good.
bob.

Posted 18 March 2017 at 14:30:36 UK time
paulh4, West Midlands, United Kingdom

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.

Posted 18 March 2017 at 15:26:57 UK time
Bob Taylor, Essex, United Kingdom

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.

Posted 18 March 2017 at 15:28:24 UK time
Bob Taylor, Essex, United Kingdom

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.

Posted 19 March 2017 at 17:02:34 UK time
Trevor Harvey, Norfolk, United Kingdom

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.

Posted 19 March 2017 at 20:09:33 UK time
Mike Howlett, Strathclyde, United Kingdom

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.

Posted 20 March 2017 at 00:34:22 UK time
William Revit, Tasmania, Australia

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

Rich
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--

willy

Posted 20 March 2017 at 08:58:48 UK time
Chris at Octarine Services, Essex, United Kingdom, chris@octarine-services.co.uk

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.

Posted 20 March 2017 at 10:10:20 UK time
William Revit, Tasmania, Australia

Chris
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--
Cheers
willy

Posted 20 March 2017 at 13:13:43 UK time
paulh4, West Midlands, United Kingdom

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.

Image

Posted 20 March 2017 at 13:17:36 UK time
john wright, West Yorkshire, United Kingdom

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.

Posted 21 March 2017 at 00:58:35 UK time
William Revit, Tasmania, Australia

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

Posted 21 March 2017 at 08:58:36 UK time
Mike Ellsmore, Victoria, Australia

The MGA master cylinder has a slow return valve in the system. See the MGAguru's explanation here
http://www.mgaguru.com/mgtech/hydraulics/ht116.htm
Hope this helps.
Mike

Posted 21 March 2017 at 12:19:45 UK time
paulh4, West Midlands, United Kingdom

Same as the MGB.

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