Mdc674lpn is correct but if you here an air hiss as you step on the brakes then the brake booster is gone. I believe you actually have a bad brake booster. It is the black drum connected behind the master cylinder. It uses vacuum from the engine to double the amount of pressure applied to the brake master cylinder when you step on the brake pedal.
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Could be air in the brake lines. Bleed air from all four wheels and then test.
SOURCE: no brake pressure
should not need to bleed module try giving the proportion valve afew good smacks sometimes this will dislodge the valve then rebleed
SOURCE: Power brakes not working
No such thing as vacuum lines to the master cylinder. You do have a vacuum line on the brake booster (power assist). When you step on the pedal do you hear a whoosing sound down by your pedal. This is a leaking Booster. Needs replacement.
When you step on the brake does the engine rpm change. This is a leaking check valve (one way air valve).
SOURCE: Brake pedal still goes to floor 2000 GMC Jimmy
It's very possible to be the master cylinder, the o-rings on the piston could be worn, letting the fluid seep back through instead of getting full pressure through the lines. Could also be the brake booster.
Try changing the master cylinder first, which is cheaper and easier, and see if there is a difference, you'll need to bleed the brakes again.
To check the booster,
With the engine off, pump the brakes until the pedal is hard, then hold pressure on the brake pedal and listen for air escaping, or the pedal goes slowly to the floor, Do that after the master cylinder has been changed, at least that will be eliminated.
Testimonial: "Hadn't thought about the O-rings on the rod thanks for the idea. Great Idea Thanks!"
SOURCE: 1997 GMC Sierra 5.7 changed master cylinder
check under your master cylinder,is there another clyinder below with one line in and one line out,if you do I found this is for the Abs to the rear,I had to replace this it was full of air,and did exactly what yours is now
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