Have a 2002 Duramax HD. 200,000 plus miles.
Brakes were working fine. Started out this AM and brake pedal very weak and goes almost to the floor. Pedal creeps back up slowly.Still have braking but don't want to drive and lose all brakes. Too many deep hollers around here.
Any ideas as to what is the problem?

Fluid is within operating range.

Thanks in advance

 

It's the Master cylinder........ Plungers are DOne Dude. Hydra- boost. CHeck Hyrdolic lines. for leaking/ Not related. Just saying