i just messed with brakes the other day, the sammie that i was working on had what sounds like the same problem that you have. it was massive ammounts of air in the lines. Try bleeding the lines, thats the common problem to breaks that you have to pump up.
So just do lots and lots of bleeding, and when you think theres no air in the lines...try some more, you will be suprised how much air can be in a line.
It might be somthing else, but you might as well check the simplest thing.
get some break fluid and go to do.
good luck