The MGA has one master cylinder unit for both the brakes and the hydraulic clutch. It is carried in a bracket on the heater shelf in the engine compartment. The bracket also serves as the pedal box for the brakes and clutch. A simple and elegant design that is very easy to service. However, there is one big failing. Because Dot 3 and4 brake fluid eats paint with ease, you typically see the paint on the heater shelf ruined by spilled brake fluid or fluid that leaks from the unit – and they all leak eventually. This then leads to rust – besides being ugly.
Another issue that is common with these units is that the holes and pins that secures the top of the brake and clutch pedals to the input rods driving the fluid in the master cylinder wear. The pin can get worn down, but worse still the hole can become hugely enlarged and eventually break. This seems to be most common on the clutch pedal. My clutch pedal and pin were in bad shape. Check yours to see.
I did not bother trying to re-sleeve and rebuild the master cylinder. They are not very expensive so I used a new one. Remember though to keep the brass fittings on your old master as they are needed on the new one. As for the worn pin and enlarged pedal… The pin is easy to replace. For the pedal I had to weld it up and re-drill. There are some contortions getting the pedal box/master cylinder support bolted in – it is best to have the steering wheel out of your way. It is all pretty straight forward though. In any case, it all came out well I think.