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MG MGB Technical - Brakes locked.

Hello all,
I used to frequent the forum some years ago but haven't been on for while as the car has been off the road. However, I've been recommissioning and have a problem with the brakes.
The car (71 GT) had worked OK on a quick trip round the block, but when I checked the brakes I found that both rear wheel cylinders were weeping.
I replaced those, but couldn't get a good pedal - long travel and then it would pump up to around half the intial distance. This was with either the usual pipe with a valve on the end, or using a MightyVac.
I thought I'd see what it felt like when actually moving, but once I turned the engine on the pedal is very solid with no travel and all the wheels are locked up (I checked each one in turn).
Presumably the servo has something to do with this, as the pedal was moving before the engine was run. Has anyone else had this problem, or similar? I've owned the car for 37 years, not seen this one before!
D Smith

It certainly sounds like something is holding pressure, either the master cylinder or the servo.

If it's the master cylinder, then cracking open the pipe going into the servo should relieve the pressure. Obviously using a piece of rag to catch any fluid that escapes.

If there is still pressure in the system, then it would point to the servo. Try cracking open the pipe exiting the servo and that should then relieve the pressure.

My car wasn't fitted with a servo, as it was an option at the time - 1967. I had considered fitting one, but thought better of it. I decided it was just one more thing to go wrong and the car stops fine without it.
Dave O'Neill 2

Hi Dave, thanks for the reply.
After posting, I did try loosening the servo outlet pipe expecting a spurt of fluid, but nothing. I then opened the inlet pipe and got some pressurised fluid out. I could then spin a wheel and the pedal also felt normal.
Now, with the engine off, the pedal goes solid after one press and the wheels are locked. Loosening the servo inlet pipe again releases them.
So it seems to be the master cylinder, but I'm not sure why the servo has triggered it. It's the original square type master cylinder and I don't remember having rebuilt it before, so I can't complain really. I'll pull it out and see if there's a wear lip in the bore before deciding on reseal or replace. I'd like to keep the original if I can.

Dave.
D Smith

Servo issues can cause this, my roadster tends to do that in very hot weather with the high and hard pedal you describe and I have to tap the brake pedal to release them. Not uncommon and apparently caused by the air valve in the servo sticking. However opening the outlet pipe should be enough to release them. If releasing the inlet pipe releases them then I don't see why releasing the outlet pipe doesn't, after all it will be pressure in the outlet that is holding the brakes on. Forgive me, but are you sure you have identified the pipes correctly?

What happens with the engine off? If it has been running recently then you would normally have to operate the pedal several times to release all the vacuum in the reservoir. If the first press with the engine off locks the brakes you may have to open the hydraulics and repeat several times. If it doesn't do it with no vacuum in the servo then it sounds like the servo. If it still does it then it sounds like the master cylinder.

Another possibility is the master cylinder piston has to come all the way back before it clears the bypass port to allow fluid to expand and contract and release pressure in the brakes. Is anything stopping that or the pedal coming all the way back? Having said that it's usually only a problem when the fluid has expanded, it seems odd that one press of the pedal is enough to do it, and only after changing the wheel cylinders.

A third possibility is the slow-return valve in the master cylinder outlet (inside the cylinder). This has a large passage to allow fluid through freely when you apply the brakes (first image), but only a very small passage to allow the fluid to come back (second image). Perhaps with the work on the system that return passage has become blocked. But again, opening either the inlet pipe or the outlet pipe on the servo should release the pressure.





paulh4

Hi Paul, thanks for the detailed reply.
I had traced the pipework by feel where it goes behind a shield in front of the heater, but my fingers must be due a recalibration as I was convinced the pipe going to the 'nose' of the servo was the inlet (which didn't seem right to me). Having now found a cutaway drg online, it isn't!
It makes a bit more sense that the servo would stick on somehow. I suspect that I didn't undo the (real) inlet pipe enough to release pressure.
I'll have another look at it when I get home, go through your various points and report back.
The servo is the original but was rebuilt, albeit a long time ago, when fluid was finding it's way into the cylinders.
D Smith

Yes, out of the end of the cylinder :o)

When the servo comes into operation the diaphragm pushes on a rod which pushes on the piston inside the servo cylinder, it is that which provides the increased force. In doing so it blocks a hole through the middle of the piston (image1).

When the brakes pedal is released the diaphragm should be pushed back by the spring, opens the hole in the middle of the piston allowing fluid to flow back to the master (image2). Most of the coloured arrows show how the air flows but you should get the idea.

So it looks like the servo is stuck or sticking operated, what happens when there is no vacuum in the reservoir should indicate which. With no vacuum fluid pressure should come from the master and pass freely through the servo piston to the brakes.





paulh4

When a car has been inactive for a long time hydraulics are usually a casualty. I would be stripping and checking all components, we are talking brakes after all.
Clutch is likely to have similar issues, though if it fails it is merely inconvenient.
Water pump seal also suffers quite often, but doesn't show up till after a bit of running.
Paul Walbran

I've not had a chance to get back to this yet, my wife had a knee replacement op last week, so I've been otherwise occupied... Hopefully do a bit more on the weekend.
I agree about checking all the components in the braking system. Other than the rear cylinders, the system is all original as far as I'm aware, although the servo was re-diaphramed by me some years ago now. Replacing the pipework wouldn't go amiss either if I'm honest.
D Smith

A locking on of the brakes, with remote servo as the cause, can usually be proved by removing the vacuum pipe from the manifold and plugging the connection.
The "push" on the slave cylinder is achieved by admitting atmospheric air at normal pressure behind the diaphragm, which has a lower pressure on its other side. Take away the lower manifold pressure, and then the big coil spring allows the diaphragm to return to its equilibrium state, and thus opens the piston valve allowing hydraulic pressure in the system to be released. Without the servo, the brakes will still work quite well, as the remote servo was an "add-on" to the same basic system.
Allan Reeling

A pal's V8 had locking-on so badly when touring that we removed the vacuum hose from the servo and plugged the open end of that with the pointy end of a spare spark plug. Probably easier than sealing the manifold port.
paulh4

I've spent a bit more time on this now, it appears to be an 'evolving situation '...

1) Engine off, pedal with an inch or two of movement and no need to pump up the pedal as before (air seems to have gone now...), hydraulic and air systems tight. Front wheel clear of ground to check for locked brakes.

2) Start engine, pedal moves once normally then goes hard, engine off. Front wheel locked.

3) Bleed off fluid from servo output union, front wheel now rotates.

4) Repeat step 2, but this time pull servo hose off manifold, front wheel still locked.

5) Repeat step 3, but now pedal goes solid even with engine off!

So now it seems like the master cylinder, rather than the servo after all. I thinking I might get the wherewithal to make up a servo bypass pipe and see what happens with that installed and confirm it one way or the other.
D Smith

You're saying singular "front wheel." Is it just one wheel or are both front wheels locked? If just one, I wonder if the hydraulics are OK but the flex hose to the caliper is constricted. That may have already ben suggested and eliminated but I can't remember so just checking.

Jud
J K Chapin

Hi Jud,
I jacked all four wheels previously to check and they were all locked, so I just got the front nearside in the air for now.
D Smith

The servo reservoir will hold the vacuum for some time which could explain 5. With mine (both when I had two) I have to pump about half a dozen times with the engine off to 'empty' it, with the servo (I presume) clanking progressively less each time and the pedal getting harder, but not as hard as when the brakes are applied.
paulh4

My money is on the little short operating piston in the booster--they rust up and stick--moisture gets sucked in through the filter on the booster--they all do it eventually
William Revit

"So now it seems like the master cylinder, rather than the servo after all."

Incidentally if your previous tests disconnecting first one pipe then the other have the same results as before i.e. one releases them but the other doesn't then it's not the master cylinder. If it were then disconnecting either servo pipe would release them.
paulh4

Got it Dave. I didn't see the first part about all 4 wheels. Jud
J K Chapin

Had another look this evening (the wheels are still locked from the last test four days ago!) This time, I undid the servo inlet pipe completely to check on Paul's last comments. Although I had done this previously, I'd only undone it a turn or two thinking that would be enough to release the pressure. However, the wheel did not release even with the union fully off. The wheel would turn only after undoing the servo outlet.

I'm trying to picture why the servo would cause this, as with both sides of the diaphragm open to atmospheric pressure the servo should be in bypass mode and the centre of the servo piston open to allow fluid through - perhaps the hole is blocked, or as William suggests, the pin operated by the diaphragm is stuck in the bypass? Probably time to have look inside I think.
D Smith

Look at the first image I posted on 18th May at 14:21.

When you apply the brakes the air valve opens to allow air to the back of the diaphragm, this pushes the main piston to apply enhanced pressure to the brakes, and in doing so it blocks the hole through the middle of that piston.

When you release the brakes the air valve should close sealing off the air inlet and opening the lower part of the valve. This allows vacuum to get to the back of the diaphragm again, equalising the pressure (vacuum both sides) and the spring pushes the diaphragm back, see the second image in that post. This allows the main piston to come back releasing brake pressure.

Usually the cause of brakes sticking on is the air-valve sticking, as Willy on 25th said, not allowing vacuum to replace the air at atmospheric pressure at the back of the diaphragm. Not uncommon, sometimes removing it and coating its piston and cylinder in brake fluid is enough, but in some cases the cylinder has to be polished with fine wet and dry to 'relieve' it slightly.

I appreciate your wheels are still locked after four days but I last drove mine four days ago and just now it still needed three pushes of the brake pedal to release the vacuum and the audible wheezing die away. So the normal parked state has both sides of the diaphragm subject to vacuum, not air at atmospheric pressure. This is when the spring should push it back, but the back of the diaphragm can only get the same vacuum as the front if the air valve opens the vacuum supply to the back. Without that vacuum at the front is still pushing the pushrod and the main piston applying the brakes.

It could also be the diaphragm push-rod sticking in its seal in the body of the servo. At the moment I'm not sure what test you could do to see which of those two possibilities is the cause.

paulh4

Dave you're assuming that the front of the diaphragm is open to atmospheric air pressure, BUT, the vacuum side of the diaphragm has one non return valve fitted to the "can", and another one fitted to the inlet manifold. They are there to maintain a vacuum and thus assist braking even if the engine has stalled. Maybe this NR valve is faulty and retaining just enough vacuum to prevent the piston valve opening?
Allan Reeling

Different view, FWIW.

The NR valve is there to retain a vacuum in the can, on the hydraulic side of the diaphragm, to maintain assistance in the event of an engine stall as you say.

The valve _could_ be faulty, either allowing vacuum to leak away (which would need a leaky hose or a leaky NR on the inlet manifold), or be blocked. Leaky would release the brakes after a short time with the engine stopped, blocked would prevent the diaphragm moving to assist braking and wouldn't lock on.

Something is keeping the diaphragm pushed towards the hydraulic side, either because the air-valve isn't allowing vacuum to get to the back of the diaphragm when the brake pedal is released, or the diaphragm or push-rod is physically jamming in the operated position.
paulh4

Spot on Paul, I was having a Long Covid brain fog.
Allan Reeling

No worries, I fear I'll be claiming age limitations soon :o)
paulh4

Yes, sorry - vacuum on both sides of the diaphragm, not atmospheric pressure. As politicians like to say, 'I mis-spoke'...!
So yesterday I started off by trying the brake pedal and it felt normal, not going solid on the next push (I had previously released the presure via the servo outlet union). I started the engjne and again, the pedal felt quite normal, and no locked wheels this time - it appears to have cured itself, not that I trust it!
I will take a look at the air valve and see if corrosion there was a cause, as Willy suggested.
Alan, Paul - you mentioned a valve on the manifold, but I only have the air valve on the servo itself. Or is the barb on the manifold actually the second valve?
D Smith

The manifold piece is described variously as 'adapter' or 'one-way valve' depending where I look, the servo piece generally as 'one-way valve'.

I can't recall what either of mine (V8 and roadster) looked like when/if I had the hose off the manifold.
paulh4

Apologies for the intermittent nature of this thread, but I’m not getting a lot of time to spend on it at the moment. However, I did remove the servo air valve cover yesterday and found a few concerns. Firstly, fluid in the chamber:-

I mopped that up, removed the plate and looked for the piston, then realised it was fully ‘up’:-

I can push it back down, but on the next operation it goes up and stays up. There looks to be some black gunk in there like carbon, but might well be old rubber from the seals. So that needs a good clean out. I’ll check it for wear and get a seal kit. Thanks for the replies so far, very helpful.





D Smith

I've removed the servo and would like to get a seal kit on order ahead of stripping it down, but not sure what bore size it is.
I found a table of servo part numbers and their bore sizes that Allan posted on the MG Experience site a few years back, but the numbers on my servo don't match the LR prefixes in the table.
Mine has '232 656C' cast in and '4257-812-4' on an aluminium tag. I can measure it once it's in bits, but I'm interested to know if you can tell the size from the cast numbers.
D Smith

I found a lot of conflicting information about bore sizes, change dates, part numbers and ratios.

Measuring is probably the safest way to go.
paulh4

I'm pretty sure the cast in number is the casting number, the type 6 identification is on the aluminium tag.......assuming it's original. Phone Peter at North West Classics.
For General Enquiries email northwestclassic@btinternet.com Tel +44 (0) 161 799 2653 Workshop 07875674565.
Allan Reeling

This thread was discussed between 17/05/2026 and 15/06/2026

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