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MG MGB Technical - Total Brake Failure

Fun at the weekend....after a longish run I had a total brake failure, pedal to the floor, no response at all! After some Sweeneyish cornering I nursed the beast home. As there is still fluid in the cylinder, it looks like a master cylinder failure. Would a servo fail cause this? or would it just make braking very heavy. I would have thought that the master would fail progressively rather than no brakes at all? Any simple tests I can do, and as I'll be stripping most of the system down, are there any simple (cheap?) upgrades I can carry out at the same time?
P M Gregory

The internal seal in the master cylinder will have failed. No loss of fluid, but no braking action either. Cheers - Dave
David DuBois

You would barely notice the loss of servo action, it was optional at one point with the remainder of the braking system being the same on both cars.

How many times did you operate the brake pedal? If only a couple of times it could be one of several things wrong, pump the brake pedal to see if the fluid level lowers. If no visible leaks then the problem could be in the remote servo i.e. filling up with fluid. If repeated operation of the brake pedal now shows no lowering of fluid level then it almost certainly is the master pressure seal failed.

No real upgrades to the hydraulic system without significant changes to fit an unboosted North American spec dual system, or very major changes to fit the later boosted dual system that was on all cars.
Paul Hunt 2010

Thanks, as always simple clear advice.

Pat
P M Gregory

Another quick one on this, as mine is 1980 it has a tandem master with pressure differential warning. From my limited research it would appear that I should have had some braking action available, albeit either front or rear. On disassembling the master, the pressure differential unit is stuck inside the body and is badly corroded, hence my need for a repacement master. Would this cause total failure, or should I investigate further into the cause. The pedal (when cool) was hard again. Could ballooning of a soft hose cause this, as there are no obvious source of leaks.
P M Gregory

I had the Diaphragm go in the Servo leaving me with no brakes...
K Harris

I jumped in my B (single inline plus servo) this morning to go to the shops and the brake pedal went straight to the floor, I checked the master cylinder and it was empty, but there was no fluid in the car. I replaced a rear brake line and the rear brake flexi-pipe a couple of weeks back so I first checked that nothing was amiss here - all tip-top and no signs of leaks. Next I checked the rear wheel cylinders and again no signs of leaks. Similarly the front hoses and calipers showed no evidence of leaking, so I narrowed it down to either the master cylinder or the servo.

I topped up the master cylinder and after a bit of pumping some pressure returned to the system, I pressed firmly on the brake pedal to see if I could see any hint of leakage from the master cylinder and a single drop fell onto the driver's mat. I removed the pedal box cover and could see where brake fluid had been oozing out from the dust seal. I removed the master cylinder and found a massive tear in the outer seal. Incidentally I noticed that the clutch cylinder was also oozing slightly (both are the original metal Lockhead units).

As for the lost fluid, there was a lot of brake-fluid soaked 'gunge' under the master cylinder and a wipe down with a rag showed the fluid had crept along the bulkhead to where the oil pressure pipe passes through the bulkhead. My driveway slopes downward so the car is always parked-up nose down and it appears the fluid had pooled behind the pedal box and crept along the flat top of the bulkhead.Strangly there was no signs of brake fluid under the car and it seems it was absorbed by the now damaged paintwork.


Mike

""Strangly there was no signs of brake fluid under the car and it seems it was absorbed by the now damaged paintwork.""

Thats why I use the Silicon fluid (even though it isnt a Brake Fluid) at least it doesnt damage the paint...
K Harris

The pressure differential switch is operated by a shuttle which could corrode, but that shouldn't transmit pressure *to* the fluid, only sense what pressure is in the lines. However severe corrosion may have crept from the shuttle to the pistons and siezed those as well. The circuit closest to the pedal *should* be the front, and if this piston had seized it would prevent pressure being applied to both front and rear. Difficult to imagine this suddenly happening though.

The MGB remote servo only adds very light assistance and is hardly noticeable when disabled, but I understand the later dual master with integral servo is different and loss of servo can have a significant effect on retardation. So maybe it is the servo that has failed and not the master.

Brake fluid does dry, unlike oil.
Paul Hunt 2010

My take on the operation of the unit (thanks Haynes!) is that if there is a difference in pressure between front and rear, the shuttle is pushed across and seals the faulty circuit, allowing brake pressure where there is resistance, Is this correct?
Pat Gregory

I can't see that in my Haynes, for either the integral or remote pressure differential switches. The section on the remote pressure switch (used only on American spec cars with the unboosted dual master) does say it closes the *switch*, is that what you are thinking of? If it were the case you would never be able to bleed the brakes with the pedal, and for the remote type it is only because pressure is still on both sides of the piston when the shuttle valve is to one side or the other that you can use the pedal to centralise it again. The integral type is a bit different, by unscrewing the switch you can prevent it latching during bleeding, so even though the shuttle probably does move during bleeding it moves back again under spring pressure when the fluid pressure is released. It is only when the switch is screwed into position that it will latch and hold the shuttle to one side with a pressure difference. Unscrewing the switch should recentralise the shuttle, but again if it sealed an open hydraulic circuit you wouldn't be able to bleed it.
Paul Hunt 2010

I looked at the diagram and guessed at the operation. The shuttle appears to be ported on both circuits so any diiference in pressure would push it across, not sure if it seals or is just used for the warning light though.

pat
Pat Gregory

Just to finish up. Opened the servo & it had about 100ml of brake fluid inside, so i suspect this was a problem that the previous owner kept topping up. Rebuilt the Servo & installed new master cylinder. bled the brakes and they're better but not great. I'll probably now replace the whole system for safety's sake.Including slaves. I noticed that the bleed nipple for the o/s rear is smaller than the n/s, is this standard or does it point to a previous failure? Anyone know a good mortgage advisor!
Pat Gregory

The rear slave nipples should of course be the same size, which are smaller than the fronts. Could just be a single slave replacement that came with a different nipple.

Brake fluid leaks in the single-circuit remote servo can empty the master!
Paul Hunt 2010

This thread was discussed between 22/06/2010 and 06/07/2010

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