Mrmerlin

try this go to the brake pedal and screw the push rod into the clevis ( increasing the clearance) one turn and the drive the car , take an IR gun with you to see if the heating is happening if so then remove another turn of the push rod do this a few times to see if it helps.
You should be able to feel if the brake pushrod is free then pushing the MC

Was the old MC leaking?
did it leak into the Booster?
Have you removed the booster feed line to see if there is anything in the bottom of the booster, ( to remove the vacuum line with the engine off step on the brakes till there isnt any more boost then pull the line out of the booster, take piece of speaker wire and drop it into the booster see if it comes out wet

Dannyfumi

Update-
Nothing has solved the problem so far! I replaced the master cylinder and booster, adjusted the brake pedal to WSM specifications. What else could be causing this the calipers, I just have a hard time believing both calipers would fail at the same time? Any thoughts, getting very frustrated with this problem, I want to drive my car!


Thanks-

Dan

Mrmerlin

what year is the car your working on??? please add it to your signature

Lizard928

Remove the rear calipers and open them up, it is possible that there is a load of gunk build up inside of them.

Dannyfumi

the car is 87s4 auto. Ill give removing the calipers a shot. Thanks

Mrmerlin

really what was the last thing you fixed B4 all of this happened

Danny can you please add the car info to your member info

Dannyfumi

The last thing I touched was safety neutral switch, a couple months before this mess began.


Danny

ZEUS+

How long do they stay locked ? When locked: If you crack open a bleeder and the rotor spins, the pressure restriction is pre caliper. With bleeder open and and rotor locked, caliper is seized.

Dannyfumi

Its pre caliper, when I open the bleeder valve it frees the wheels. So where does that lead me to?


Danny

ZEUS+

That eliminates calipers and e-brake. Loosen up the master and see if they free up. Have to isolate one step at a time.

Mrmerlin

after the master then i would suggest that you replace the bias valve thats screwed into the ABS unit for the rear brakes make sure to get the same one it has a number on one of the flats 55,33 18

Brad W

Danny I would look at the ABS system. I think there is a control valve that is not releasing the rear calipers. I am headed over to Rogers and I will look at one of his older units. Maybe we can change it out this weekend and try that. It sounds like the shotgun effect has not worked out yet so this can not hurt.

Brad W

Read # 5

When the vehicle’s state has gone into the ABS mode, every part of the hydraulic control unit so that the wheels can not be locked. The isolation solenoid valves are normally open valves, while the release solenoid valves are normally close valves, they can turn to the opposite state under the control signal. ABS control takes effect in five phases:


1. Building up the pressure

This phase happens at the beginning of braking. Braking pressure in brake calipers is increased proportionally to the brake pedal force. Consequently, the speed of wheels decreases.

2. Decreasing the pressure

When the wheel acceleration continues to decrease below a threshold determined before, the release solenoid valve will open to decrease the brake pressure in order to avoid the wheels from being locked.

In this process, brake pressure decreasing mainly depends on the effect of hydraulic pump. For the hydraulic pump may not pump brake oil immediately, the redundant brake oil will flow into the low pressure accumulator for temporarily conservation. The restrictor is a hydraulic unit for reducing pulse of the pump.

3. Holding the pressure

In this phase, the isolation and release solenoid valves are both close so that the brake pressure can not change.

4. Increasing the pressure

When the wheel acceleration continues to increase over a threshold determined before, the release solenoid valve will close and the isolation valve will open to make the brake pressure increase.

Phase 2~4 will repeat several times until the vehicle speed reduces below 3km/h.

5. Completely releasing the pressure

This phase happens at the end of braking. At this time, the driver releases the brake pedal. Therefore, the solenoid valves return to their normal states and the brake oil flows back to the master cylinder.

Pete L

I had a similar issue for years on my 86 where occasionally the brakes would drag. I adjusted the MC rod as suggested above and ended up with a less then firm brake pedal. But, it dod cause the drag to cease and desist/

Strangely enough, I replaced the vacuum bushing going into the power brake booster and miraculously the problem seems solved (firm pedal, no more brake drag).

Just a simple idea as the bushing was just a few $

Mrmerlin

Pete thanks for the info do you know the part number of the part that you replaced and where exactly did it go??
Thanks Stan