how do you tell if your brake master is bad?

[vardenafil]

How do you tell if your brake master cylinder is bad? When the truck is off the pedal is firm and hard. As soon as I turn the truck on the pedal gets super mushy. I can lock up the wheels but only after the pedal reaches the floor. I have to pull the pedal back up with my foot to get it to return to the normal position. I pulled the master off and did not notice any external leaking into the booster or anything like that.

here is what ive done so far to the brake system:
-I replaced all the soft brake lines on my 72 k20 with braided brake lines.
-I replaced the calipers and pads on all four corners.
- Ive bled and re bled the truck multiple times and no air comes out.
- Ive adjusted the rear parking brake calipers they have almost no slack.

The only thing I havent replaced is the master and the brake booster and the hard steel brake lines.

Does this sound like a master problem? or is there something im missing?

[CarlsLQ9SS]

Are the bleeders on top or the calipers? I've seen new calipers stamped wrong and it puts the bleeders on the bottom and makes it impossible to get the air out of the system.

[vardenafil]

The bleeder screws are on top. The only thing that makes me think it might be the master is no bubbles come out when I bleed.

[HotWheelsFan]

Sounds like you have rear disc brakes, do any of each corners have excessive distance from the pads to rotors before pressing the pedal?

If no air is in the system, sounds like an adjustment situation, or something out of spec. if you have not messed with the pedal to master assembly.

Unless you had the same problem before working on brake lines and calipers...


Bob

[vardenafil]

I just bought the truck like 2 months ago. the brakes never worked right. I had the same symptoms when i bought the truck. I was told it was do to a leaky rear wheel cylinder. I did not see any leaks when I swapped the drums out to discs. I figure if it was an easy fix the previous owner would replaced the 8 dollar rear cylinder. So i assume something bigger is going on.

one of the previous posters asked if the brake bleeder screws were on top. the bleeder screw is on top but at a slight angle (red line). Do i need to move the caliper bracket a notch or two to make the bleeder screw vertical like the blue line?

[vardenafil]

[Rust_never_sleeps]

Weird that it's sticking down.
Did the m/c spring back normally when you had it off the car? That's the biggest spring in the deal, so is it binding in the bore? (or somewhere on the linkage)

Boosters can fail in weird ways and stick down, I am told. I'd guess the vents stick down or the return spring goes bye-bye, but others here would have far more to offer on that topic.

[vardenafil]

Originally Posted by Rust_never_sleeps

Weird that it's sticking down.
Did the m/c spring back normally when you had it off the car? That's the biggest spring in the deal, so is it binding in the bore? (or somewhere on the linkage)

Boosters can fail in weird ways and stick down, I am told. I'd guess the vents stick down or the return spring goes bye-bye, but others here would have far more to offer on that topic.

there was some binding where the rod goes thru the fire wall to the brake pedal. the rod wasnt aligned in the hole. i enlarged the hole slightly on the bottom so it wouldnt hit. when i bleed the truck the master feels like it catches and releases. but im not sure if its the linkage or the masteritself

[Ironangel]

Having to pull your brake pedal back up by hand is an indicator that the return spring isnt working or gone? There's something not right between the pedal and the booster. That rod shouldn't touch anything and should be adjustable. What year and model is your truck? On the rear I noticed the E-brake cable is missing. Im wondering if those aftermarket E-brakes have to be adjusted before bleeding to close the distance between the pads and rotor? And rotating the calipers one hole down would favor the bleeder position by putting them on top. Here's a thread with pics showing the return spring http://67-72chevytrucks.com/vboard/s...d.php?t=761902