MGB Register Forum

Brake fluid leak traced to master cylinder. Replaced master cylinder and now cannot bleed all brakes. Left front seems to bleed ok. Right front will not get fluid even with vacuum. Rear don't seem to get fluid but have not pursued that yet. Also, with left front bleed screw open, pedal goes to floor. With that screw closed, pedal goes about halfway down then sounds/feels like it hits metal. Did I get a bad replacement part? Any other thoughts?
Walt

Assuming you got a master cylinder from one of the main suppliers, I would suspect air locks causing problems.
I would open the bleed screws, fill the reservoir and leave overnight. You may find that gravity sorts it out.
Bleeding the rear brakes may also be enough to shift any air pockets.

Ian F

Does your car have dual circuit brakes? If so, there can be issues caused by the imbalance switch in the master cylinder. I normally refer to Paul Hunt's site http://mgb-stuff.org.uk but in this case he states that he has no experience of the dual circuit brakes. However he puts down his thoughts and I copy them below:

For dual-circuit systems later Workshop Manuals and Haynes say to bleed the caliper nearest the master cylinder first, then the other caliper, then the rears - order not specified. These, including the brake-failure shuttle switch, are outside my experience but I can't see how they can be pedal-bled very easily as the circuit not being bled will still pressurise and stop the pedal moving to the floor, which will reduce pedal travel, and hence the amount of fluid that can be pushed through the system that is being bled with each operation of the pedal. That may be the reason why the calipers are bled first, as any air in the rear circuit will allow the pedal to travel further when bleeding the fronts (and the front brakes are more important than the rears), and again releasing the handbrake or even disconnecting the handbrake cable from one of the back-plates may help. Continuous-flow bleeding as with a Gunson's EeziBleed should avoid both these issues, as well as making the job much easier on all systems. However on both my (single-circuit) cars I find that whilst the EeziBleed is perfectly adequate for the rear brakes, if the fronts have been dismantled and air has got into the system additional steps are needed, as after using the Gunsons alone I s till get a 'long' pedal. This can be 'pumped up' but after only a few seconds that effect is lost and the next application results in a long pedal again (the effect of the restrictor valve inside the master cylinder), which can be a bit disconcerting! I get my assistant to press down hard on the pedal while I rapidly open and shut each caliper bleed nipple in turn. The much higher pressures, and hence flow when the bleed nipples are opened, seems to blast any remaining air out of the pipes. Thereafter the brakes have always been as expected.

Dual-circuit systems need special consideration as depressing the pedal with one circuit open and the other closed is likely to activate the 'imbalance' detector and bring up a warning light on the dashboard. The UK only got dual-circuit brakes at the same time as the master cylinder with servo, and on these the balance switch is under the master cylinder. This can be partially unscrewed, which although the warning light may come on as you depress the pedal (if the ignition were on) it will reset itself when you release the pedal, and you screw the switch back when you are finished.

Thank you both for your responses. I will get back to the brakes later this week and I will try your suggestions. Will let you know what happens.

When I bled the brakes on my dual brake system MGB I used a pressure bleeder. There's no need to to use the brake pedal doing it this way and the shuttle valve isn't distributed.
Better still is a vacuum bleeder.
The disadvantage of using the brake pedal method is that under normal use the brake pedal never goes full travel unlike the clutch and corrosion can set in on the part where the pushrod and piston don't normally go. Pushing the brake pedal right down means the piston and seals go along that part and any corrosion can damage the piston seals

It woulkd be great if you could clarify whether you have single or dual circuit brakes.

Withe the brake lines disconnected and plenty of fluid in the resevoir get someone to press the pedal down gently. Does brake fluid come out of the union/s at a fair pressure ?

Short update. Sent new MC back as it seemed to have an internal issue, received another. Installing tomorrow. It is a dual circuit system with a booster. I'm not familiar with any shuttle valve. What is that? There is a switch mounted under the MC which appears to be some kind of pressure sensor. Going to buy/rent a vacuum pump to bleed after the install as the explanation regarding using the pedal to pump/bleed with 2 separate systems not working seems reasonable. Will keep you updated and thank you all again for the responses and assistance. I really do appreciate all of you.
Walt

In short the shuttle thing is measures the pressure differential between the front and rear circuits. Its that which would bring up the pressure warning light as I understand it.
An awful lot seems to be written about it without context, to the extent that i was convinced for weeks the sole reason I was unable to sucessfully bleed my MC at one time was due to my inability to set up the shuttle valve. Completely wrong, the MC was just knackered.
In my expereience if the MC is bad then it needs replacing and if it's good then you dont need to give the shuttle thing a second thought.

Finally got the issue resolved. I missed the part in the manual that says to unscrew the PD switch 3 1/2 turns. The switch is mounted on the bottom of the master cylinder. Once that was unscrewed, the brakes bled normally and all is well. Never in the 50 years I've bled brake systems ( never an MGB before) has that issue come up. Simple fix but very frustrating to find. Thanks again for all of your suggestions and input.
Walt

I don't think our American cousins got the PD shuttle valve and switch on the master cylinder, they had a separate valve on the bulkhead to do the same thing. Nearly fifty years ago we had those shuttle valves on tractors, they enable you to have two brake pedals with each one operating the brakes on one side, this enable the driver to make much tighter turns. When the brake pedals were locked together for road use the shuttle valve simply balanced the brakes.