'90 auto w/383 and SR intake

Brake mods: C5 front calipers w/13" slotted rotors, drm bias spring, brand new master cylinder and booster (metal).

This is a project car I've been test driving and the brakes are not right, they never have been. The brake pedal does not want to return to upright position immediately after pressing it and taking my foot off. When I take my foot off the pedal I can still feel the brakes pulling and the pedal very slowly comes back up, and sometimes I have to help it a little. The brakes are also mushy, but once they catch they stop the car good. Also the eniter system was taken apart, exept for the abs pump, during the rebuilt of the car, and the system was open for several months. I have bled the brake system per the helms manual numerous times, both with a vacuum pump and manually with a helper to pump the pedal. I can see no visable leaks on the calipers, and the car does not pull when the brakes are pressed. I do get a code set and that code is 75 - lateral accelerometer, I know where it is and it is plugged in, but I have no idea what it does. This is the only code.

Please give me some ideas as to what to look for.

 

Have you run through the Brake Diagnostic Chart (pp 5-3 of my '88 FSM)??

It some suggested causes for "Brakes Slow to Release".

 

Didn't even see that, I was focusing on the abs system. But according to that diagnosis, it is either the master cylinder valve (I doubt it because it is brand new) or the bias spring is not installed right. I will pull the plunger out of the mc and see if the spring is still installed properly.

 

Isn't there a procedure for adjusting the brake actuator rod to keep this from happening? .....
Since a lot of 'stuff' has taken place around the components of this system it may be a good idea to check that.

It's pretty vague but I seem to recall reading something about the rod has to be adjusted properly so the MC piston can come back far enough to relieve pressure in the system.

Just a guess, of course.

 

I would check the rod adjustment before I'd do anything else. I did the GS 13" conversion, I adjusted the rod and they work great. It's not hard and it doesnt take that long.
Good luck.

 

I actually did the rod adjustment when I changed the booster. when I did the upgrade no further adjustments were nessesary.

 

I went back and re-read your post....

I believe the key data here is "the system was open for several months".

IIRC, there is a procedure that requires a Tech I or Tech II (can't remember) that cycles the ABS solenoids, activates the ABS pump and opens/closes the valves in the ABS unit.

IIRC this is the way to get the air out of the ABS unit. Again, IIRC, this is a problem that has been encountered by folks before.

Even with a pressure bleeder, the ABS pump and components do not allow fluid 'flow thru' unless the ABS unit is 'cycled'. Therefore all the air can't be removed from the system unless the valves are cycled and the pump is running.

 

Ok so how do I do this, and btw what does IIRC stand for? Would it require going to the dealership (not going to happen)? Or might the local auto parts store have a "Tech I" or "Tech II". I hear the ABS relays click when the key is turned, and should get an ABS code if the internals are not working I would think.

Also I believe the rod is close enough to not cause this, but I will check that as well for the people that suggested it.

 

Went to the local Chevy dealer today and they said this was most likely the culprit.... but no longer had a Tech1 because it was "old and outdated" and the Tech2 would not work on my car.



So.... Is there any other way to fix this problem with the ABS? Or am I just screwed.

 

IIRC= "If I Recall Correctly' .... which I don't do near as ofter as I used to.

The anti-skid braking system uses the wheel tach generators as the reference to know when to cycle. WHen one or more wheel(s) turns slower than the others the microprocessor decides which wheel needs to have a 'pulsing' fluid pressure applied to it.
I believe that 'lateral accelerometer' also supplies input to the ABS microprocessor as well. So, the system may not be operable with that sensor defective.
Note: I could be mistaken about this and that sensor could be part of the the Active Restraint System (airbags).
I'd have to hit the FSM to find out.....I really don't know for SURE about it..............


So anyway....... what needs to happen is to trick the system into activation.....

Do the brakes work enough to lock up any wheel on a dirt or grass surface?

If so, fill the master cylinder up and drive it on some 'wet grass' or dirt and slam on brakes several times.
Skid frontwards, backwards, etc several times. Be very agressive with the braking. May want ot do this without any passengers on board.

That should activate the ABS and cause the unit to cycle.
Check the fluid each time and make sure it doesn't get low.
This 'may' move the air out of the ABS unit and into the lines. Hopefully it will leave the ABS unit full of fluid.

Now, all you have to do is clear the lines of air.......
Take it back to your garage and pressure bleed the brakes a couple of times.

Hold your mouth right and keep a couple of fingers crossed.

Maybe this isn't a 'convential' method but I'll try anything to achieve my goals.

If all the components of the ABS system are good and the only problem is air in the system, the self diagnostics will have nothing to flag as bad. There is no 'air in system' sensor. Therefore there will be no stored codes.
The engineers and programmers planned for failure of a hardware component but didn't find it necessary to advise of the necessity to bleed the system properly. You KNOW it's got air in it, as any knowledgeable person working on it would know. There's no need to overdesign to report the obvious as that adds unnecessary complexity.

Good luck with her.

You may try another dealer and hope to find one that still has the equipment to do this if you can't get it squared away..