I have just installed new calipers and pads on a Jeep GC, and I can't seem to get it to bleed properly. When I loosen the bleeders I don't get any air, and the pedal will build up, but then it drops again.

Can anyone suggest what I am missing?

 

Sounds like the problem is in the master cylinder.
Did you use the pump the pedal method to bleed?
Many times when using this method especially on an older rig the master cylinder stroke is longer than what it normally is just using the brakes. Often this part of the cylinder is rusty and will tear up the master cylinder seals resulting in an internal leak caused by depressing the pedal all the way to the floor.
Symptom is a hard brake pedal that creeps down.

 

Dido on the master cylinder. After you check/replace it, one trick I have learned is how to bleed the brakes by myself. First, fill the m.c., then get a piece of 3/16 hose long enough to reach from the bleeder to the ground. Loosen the bleeder, put one end of the hose on the bleeder, and the other end into a 1/4-1/2 full bottle of brake fluid. Make sure the end of the hose is below the fluid level in the bottle. Get in, or do it by hand, and pump the pedal dozen times or so. It is best if you have a second bottle of brake fluid to keep the master cylinder level full, and check the level in the bottle at the wheel. When the bottle at the wheel is full, tighten the bleeder, and move on to the next one. Use the now full bottle to fill the MC, and the other partial bottle at the wheel.

 

Not to sound stupid, but you are having an assistant pump up the pedal and hold pressure on it, then you crack the bleeder until the pedal hits the floor, close the bleeder and start over?

Also, what year is the vehicle/does it have a proportioning valve?

If you can get access to a mity-vac or similar, it makes brake bleeding much easier...

 

Thanks guys. I have also decided on the MC. I had this happen to me several years ago on a GM product. I guess that I was just in denial about the cylinder.
Thanks again. I will post the result.

 

Here, try one of these. Click here. With this gizmo you won't ever run the master cylinder dry, or touch the brake pedal. Works on clutches too.

 

Well, I found the trouble. I had installed the calipers on the wrong sides so that there was an air pocket at the top of the cylinders. I just followed the pattern of the ones I took off, so I do not know how the previous install got the brakes to bleed.

Just as a sideline, on this model of truck, you are not supposed to power bleed since it harms the HCM.

Someday I will learn to think first!

 

Only thing I would change here is I won't reuse the fluid I pumped out of the system I would open a new container of DOT 3 and start on the next one! Just my .02