I only have breaks sometimes!?!?!?!?!?!?!?

My car is fully tormenting me!
The car was grand, then one nite after driving for about 20 minutes, when i pressed on the break pedal it went straight to the floor! Although there was still breaking power, it was a lot less than wat it should be. I managed to get home safe enuf but when my bro tested it for me the next morning the breaks were fine again. Anyway, they seem to be fine when the car is cold but they will go soft again - ya just dont no when it will happen! Iv now noticed that when they go soft, if you pump at them then they stiffen up again??
I asked around and no-one had a clue so i phones the AA buddy and he admitted that he just didnt know?! Wasnt leaking break fluid etc so suggested the master cylinder (i think thats wat it was?) about the wee rubbers flippin in and out but apparently that just doesnt happen now?
I took it to the fella who usually fixes it, he checked and bled the breaks etc said he didnt know either but now it is leaking break fluid!! So the second fella has said i need a new calibre but if that was the case, why did no-one else no this? Is the new calibre just gonna stop the leak or is it going to fix the whole problem??? Why did it happen?
Hopefully iv made some sense and hopefully someone can help....