It is possible that the master cylinder has had it, first try checking to see if the rear shoes are fitted correctly, the return spring at the bottom of the shoes should be connected to the leading shoe and the back plate, it is common for people to put this spring on in correctly by fitting it between both shoes, this compresses the slave cylinder too much resulting in a very long pedal that gets higher as you pump it but returns to being long when released. also check shoe adjustment and try to manually bleed them, start at the furthest wheel from the master cylinder use hose clamps on the other three wheels then bleed that one, moving round until you end up at the wheel closest to the master cylinder. hope this helps :-k
