Brake problem, need help

[69K10 Tank]

Ok, so comming home from school my brake pedal went soft and all the way to the floor, but if i pumped it again i'd get brakes. Thankfully I only live about a mile from school and this started about 3 blocks from home. So once i got home my dad and I bled all the brakes and it still happens. Basically if I dont jump on the pedal, and just push steadily it will go all the way to the floor. If I push on it really hard really quick the pedal is firm, but still slowly goes to the floor. I'm kinda thinking its the master cyinder since i don't see leaks anywhere. I just got back from shucks with a new master cylinder and i'll put it on after i get home from school tomorrow (not driving, hitching a ride with a friend). Just wondering if there might be anything else or if you all think its the master cyinder too. Thanks (sorry this is kinda long and rambling, but i kinda needed to vent)