MG Car Repair: Master Cylinder, brake warning light, vacuum bleeder

Question
Hi Barrie,

I can't seem to get the front half of my MC to fill up. The rear half of the MC fills and the front brakes bleed ok, but the front remains empty. I've even put a vacuum bleeder over the hole (after removing the front hose) but all I get through is an endless amount of air.

The pressure switch is disengaged.

Many thanks in advance.

Answer
Hi Craig

I presume that you have a rubber bumper MGB with the servo mounted on the end of the master cylinder with 3 pipes coming out of it and an electrical switch underneath.

Firstly, let us deal with the basics.  Have you fully filled the master cylinder to the bottom of the filler neck?  I ask this because the plastic reservoir has a partition in the middle and just maybe you have only filled one side of it.  

If that is not the problem, then perhaps the brake failure shuttle is stuck.

Try this:  Unscrew the switch 3.5 turns but do not remove it completely.  Bleed the brakes.  Screw the switch back in again.  Turn the ignition on.  The BRAKE warning light should be ON.  Release the handbrake and apply the footbrake.  The BRAKE warning light should NOT come on and the system should now work OK.  

If the warning light does come back on, get back to me. I will also need to know if you have done anything to the master cylinder recently.