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MG MGB Technical - 80 MG B, Brake Master Cylinder

Dear Forum Members

Checked older posts, couldn't find answer to my specific "problem".

During bleeding applied much too much pressure on pedal, fall through until middle of pedal range and at the same time rear brake hose started leaking.

Replaced rear brake hose.

Now It seems master cylinder has internal problem as pressure to front brakes builds up in second part of pedal range and front brake are bled and functioning properly.

First part of pedal range does not build up pressure at rear brakes any more and bleeding at the rear can't be done (no air or fluid gets to the rear).


1. Could only master cylinder first section be damaged?
2. Is first section of pedal range actually for the rear brake?
3. If rear brake lines would be full of air would the pedal go to the middle without much resistance?
4. If rear brake line is clogged would pedal be "hard" from the beginning?

I a confused and before I order a new cylinder would like to do the right trouble shooting.
Thank you for any input.

Reto Schlumpf

It might be as simple as no fluid, your car has a 2 part reservoir, from the driver's seat it looks full, but if you check the front part (which from memory is for the rear) it may be empty. Worth checking before you delve deeper!
Pat Gregory

Thank you Pat for your input.
I did check and filled both reservoirs before bleeding. There is just no fluid going to the rear brakes and the pedal is soft in the first half.
Reto Schlumpf


Doesn't that car have the shuttle valve on the wheel well under the hood that will shut off the failed end (front or rear) if there is no resistance as if a wheel cylinder or caliper failed? If so, you may have to slide the valve back to the center. I think that can be done through the warning switch on the valve with a small screwdriver.

C R Huff

"1. Could only master cylinder first section be damaged?"

"2. Is first section of pedal range actually for the rear brake?"
>>>No. The sections of the MC are hydraulically balanced inside the MC, so both sections work together and equally unless there is a serious fault. In fault condition, the piston in the "bad" side bottoms and all remaining motion goes to the "good" side.

"3. If rear brake lines would be full of air would the pedal go to the middle without much resistance?"
>>>Yes. The piston for the rear section would bottom in the MC, using up half the pedal travel. Same if the rears were good but you had a fault in the front system.

"4. If rear brake line is clogged would pedal be "hard" from the beginning?"
>>> Usually not, because there is air in the MC or trapped between the MC and the blockage. You can attempt to bleed at the lines coming out of the MC, which should tell you if the problem is internal to the MC or downstream.

Commonly, there is wear/damage in the cylinders from age, and when any work is done that requires more travel of internal parts than they have been getting in use, as in bleeding, seals are damaged or dirt is broken free to create havoc in the works.

This system has the shuttle valve built into the MC. While this valve does not by design shut off the fluid to the bad side as people believe, it can effectively do so if there is dirt/sludge/rust build-up, which is very common. This makes it impossible to bleed the system. The shuttle is in the small bore with the plug, on the side of the main MC bore. Commonly the bore is full of nasty stuff, and all the parts are stuck in place. Once hydraulic pressure moves them, as when you lose one side pressure, the springs cannot move them back. There are small o-ring seals in there, which can be obtained as a shuttle valve repair kit. But it can be very difficult to get this apart.

FR Millmore

Oh yeah, FRM, I forgot that the shuttle valve was part of the master. Must be something else I had (or have) that has the remote shuttle. Sorry to mislead you Reto.


C R Huff

Yep, all the chrome bumper cars and most everything Brit from 68-74. At least in N America.

FR Millmore

According to the manual one has to remove the wiring from the tab (under the master cylinder) and turn the 'screw' 3 times anti-clockwise. This ensures that the piston drops down allowing the fluid to get distributed evenly in the system. Be sure not to snap the 'screw' in case it is rusted solid. Hopes this helps.

The "screw" is the pressure fail switch. It does not affect the fluid flow if all is correct, but it will keep the lamp on, and it is possible to damage the switch if it is not unscrewed while bleeding.

FR Millmore

Thank you very much gentlemen for the all the valuable inputs. It is extremely helpful. We will do the fault analysis accordingly.

One more thought/worry:
Could somehow the servo assembly be damaged too or be a part of the problem?
Reto Schlumpf

No, servo would affect both front and rear the same. Problems with servo are pretty rare, mostly just no servo action.

FR Millmore

Long day again..

Rear brake line absolutely free.
Rear slave cylinders changed (had 2 spare cylinders ready).

MC checked on workbench, seems to be in very good condition. No dirt in the pressure switch part. Pressure fail switch reset (3 1/2 turns out). Hydraulic part not dissembled (No repair kit at hand).

Put MC back in tried to bleed. Only front brakes easy to bleed and working (second half of pedal range). Rear brakes no pressure build up (neither fluid nor air).

Conclusion MC damaged (rear brakes)?
Is it repairable with a MOSS repair kit?

Thank you for a final input.
Reto Schlumpf

This shows basically how a split braking master works, and how a leak or fault in one circuit can cause a long pedal.

The 'fault' in the depicted system is in the circuit closest to the pedal, which is the front circuit on the MGB. From your symptoms the fault appears to be in the rear circuit, which will give much the same long pedal.

I'm guessing that you have blown the pressure seal for the rear circuit, i.e. the front piston i.e. the one on the left here. This could easily result in no flow through the rear circuit, as well as no pressure build-up in that circuit, as any pressure developed in front of the faulty seal will simply transfer to behind the seal, and back to the reservoir.

This also chimes with you having a problem when pressing the brake pedal 'too hard', although the average human should never be able to press the pedal too hard, but weak components (like your rear hose and as I say possibly the rear pressure seal in the master) can fail.
PaulH Solihull

Thank you Paul and all other helpful forum member thoughts. Paul, excellent analysis. Ordered initially a repair kit.
Reto Schlumpf

This thread was discussed between 14/11/2012 and 15/11/2012

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