Need some advice. I was driving down a hill pressed the brake pedal down and it stayed down. Replaced the booster last year so I thought it was the master cylinder, it's 15+ years old. Bench bled and installed new master cylinder. However it won't build up pressure? I opened the bleeder on the passenger caliber and fluid barely comes out. Pedal still "Dangles", no pressure at all just goes to the floor. I see two proportional valves along the frame that the hard lines from the Master cylinder go into. Could they both or one of them be bad? Thanks! TC

My first thought would be a bad MC. You might split the MC & booster & see if there's some free play in the threaded rod. I plug both MC ports with (2) brass plugs & if the MC/booster are good it'll have a rock hard pedal with virtually no travel(with eng idling if power).

I would agree with the master cylinder, there is a big spring inside the master cylinder that is supposed to push the rod back.
The spring could be bad, or the piston in the master cylinder could be rusted..