David,
If pumping the gives you a firm and reasonably high pedal you have not got a hydraulic problem.Period. When you pump the pedal on a drum braked car, you are progressively jacking the shoes closer and closer to the drums. When you release the pedal between pumps, the shoes do not fully retract back onto their stops. This allows them to make further progress on the next pump. In other words,if you have to pump the brakes up you have an adjustment or mechanical problem at the incdividual wheel brake(s). When you release the pedal and stay off of it, the shoes retract all the way as the cylinders bottom out. The next time you step on the pedal you have to start the process over again.
The adjuster "snails" and "masks" on MGAs are notorious for not wanting to hold an adjustment when they become old. They are often overlooked during a brake overhaul, and it's easy to see why if you add up the cost of replacing all of them..
I have also recently encountered the problem of wrongly manufactured brake shoes which couldn't be adjusted far enough to contact the drum even if the adjuster was turned so far it went "over the top" and returned to "zero". In this vein, look for drums which have worn or been machined out of tolerance and are to large in diameter to have the shoes ever reach them with one pump!!
Also, the MGA master cylinder pushrod from the pedal is adjustable for length. Make sure that you have minimal but DEFINITE free play with the pedal in the relaxed position. The shop manual covers this and I suggest you read it thoroughly.I hope some of this helps. If not, repost and we'll go from there.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon