What causes brake fluid to move from rear to front reservoir?

[72farmer]

I have a 67 C-10 that i just bought that has a brake problem.The pedal is soft and it travels a long way untill you get brakes.It has drums all the way around and a power booster.I removed cover on master cylinder a week ago and the back reservoir was dry.It goes to rear brakes right? I replaced the 2 front hoses going to the front brake wheel cylinders, bled brakes all the around and refilled master cylinder. I took the drums off and inspected brakes and parts and every thing looks new and correct.When i took the cover off of the master cylinder yesterday the back reservoir was dry again and alot of fluid ran out of the front reservoir.It has no visible brake fluid leaks to cause the back reservoir of the master cylinder to be dry.What could be causing this?