Truck Repair: brakes, piston seal, rear shoes

Question
Hi Van, I really appreciate the information and will check rear brakes. Rotors have only been turned once, but I will verify. I will also check to see if pedal rod has adjustment. One more question, is this problem normally caused by a bad master cylinder? Even though I have a new (not rebuilt) master cylinder, maybe I have a wrong or a defective one. One other thing, if you build pressure with engine off and then press brakes when starteing engine the pedal drops immediatelly to the floor. Does that mean anything? Thanks again . Dallas.-------------------------
Followup To
Question -
Van, I have a 1994 full size Chev. Blazer with 4 wheel drive. It has 4 wheel ABS system. My question deals with excessive brake pedal. I have installed new master cylinder and new front pads, rear shoes. All rotors and drums turned. Calipers and rear cylinders rebuilt. System has been bled thouroughly with new fluid. Still have too much pedal. Can you help?
Answer -
Hello Dallas,
I have been thinking of your question, and I am sure you mean you have too much travel before the brakes start to apply.
I assume your rear shoes are correctly adjusted, and even ground to fit the contour of the turned drum.
I sincerely doubt that the ABS has anything to do with the pedal travel.
If the shoes are adjusted, it might be that the rod from the brake pedal, if adjustable, is too short, or the pedal stop is allowing the pedal to come back too far.
Once the lip of the piston seal in the master cylinder passes the hole in the bottom of the reservoir, the brakes should apply quite soon.
If the shoes have a lot of travel, the pedal needs to go further to push the pistons out in the wheel cylinders.
I only mention the shoes, because on the front, the pistons stay pretty well where they are almost applied, there is no return.
If the rear drums were turned a lot, and the shoes not fitted, then they can flex as applied, which takes more travel of the cylinders, thus more pedal travel.

Van

Answer
Hello Dallas,
The last question, is the way you are supposed to check power brake boosters. Bleed the stored vacuum with several pumps, and then hold pressure. Then when the engine starts, vacuum is returned, and the booster pulls the pedal down.

All is good there, except for the part of going to the floor.

I have bought brand new master cylinders,,,well one for sure, that had a defect, so it is possible, but rare.

The rotors turned is good, and will give a nice flat surface for the pads. But the potential problem with turning, is on the rear drums. If they are turned way out to a larger diameter, and the shoes are not dressed accordingly, then only the middle, or one end of the shoes will make contact, and the rest of the shoes will bend to form to the drum.
That is easy to check if the drums and shoes are off the car. Put the shoes in the drum, and see if they fit like a glove.

Now, with ABS, it is possible that a valve is leaking in that system, allowing the fluid pressure to bleed down, not getting to the wheels. That is how the ABS works...it lets the pressure escape.

Van