Losing brake fluid but no leaks at wheel calipers/cylinders

[gdk771]

I have a 1999 Tahoe I recently purchased that is losing brake fluid. The pedal goes all the way to the floor and I thought it might have a leak at one of the wheel cylinders / calipers but when I pulled each wheel and checked, there was no leak anywhere. The brake fluid has got to be going SOMEWHERE but I can't seem to locate the source of the leak. If the master cylinder was going bad wouldn't it be leaking fluid as well? I'm puzzled on this one. None of the brake lines is leaking either.

 

[lesterl]

Assuming resivoir is empty...... Check brake lines for leaks, they tend to rot/ rust.....

 

[Slammed]

There is a brake line that runs above the rear axel that rusts and slow leaks. Also if it is the master it can leak into the booster