MG Tech – MGB Brake Master Cylinder Replacement

Roger Taylor comes to the rescue of club member Chris Adamson who discovered brake fluid pooling in the foot well of his MGB.

It didn’t take Roger long to diagnose the problem as a leaking seal on the piston of the brake master cylinder, a not uncommon situation when the piston rod becomes corroded over the years and wears a gap in the rubber seal through which fluid can leak – the only effective cure is to replace the master cylinder.

This is located under the offside of the bonnet close to the bulkhead alongside the clutch master cylinder and under a protective cover which needs to be removed using the retaining bolts. First job however will be to remove the throttle cable support which is attached to the top of the cover.

Once the cover is off, it is a good idea to remove as much of the fluid in the cylinder reservoir as possible to minimise spillages, then remove the grommet in the bulkhead panel behind the cylinder as this will allow more movement when removing the unit.

Next detach the split pin from the clevis pin which links the brake pedal to the master cylinder piston rod and remove the clevis pin. Follow this by undoing the brake pipe which fits into the rear of the cylinder and ultimately feeds fluid into the brakes.

The cylinder itself is held in place by two securing bolts top and bottom in the cover housing. Once the cylinder is out it is an opportunity to clean any excess fluid in the base of the cylinder housing.

Original brake master cylinder reservoir containers were made of metal but the only replacements available today are plastic which has the advantage of making it easier to see the level of the brake fluid which is marked on the side.

The only component that will be required from the old cylinder unit is the ‘T’ joint at the rear which links into the brake pipe. When attaching this to the new cylinder it is a good idea to fit a new copper washer to ensure a good seal. It can be tightened once in place by accessing through the bulkhead from inside the car.

Replacing the cylinder is a rather fiddly task so needs a bit of time and concentration. Once in place reassemble the pipe and piston linkages so that the system is all connected.

To replace the brake fluid and bleed the braking system will require the vehicle to be raised onto axle stands to gain access to the brake nipples on each wheel.

With the reservoir full, start with the rear nearside brake nipple and attach a bleed bottle before loosening the connection to allow fluid and air to be released. It will require a second person to pump the brake pedal to force fluid around the system removing any air. Secure the nipple when air bubbles have stopped.

Repeat this process at the other three wheels, remembering to keep the fluid reservoir topped up – it make take two or more bottles of brake fluid to complete this.

Having bled the brakes the reservoir can be topped-up and the cap screwed on before replacing the bulkhead grommet and refitting the housing cover.

Roger recommends placing a strut (a length of wood will do) between the brake pedal and the driver’s seat overnight to maintain the system under pressure for 24 hours which will help to firm-up the brake feel.

A new brake master cylinder costs around £56 and takes approximately two hours to replace.