1995 Chevrolet Lumina Euro edition. ABS Light on intermittent. GM Computer code 42. Replaced rear brake hoses, calipers and pads. Replaced brake master cylinder with a Napa re-manufactured and entire abs/master cylinder unit with a used assembly after that.
I have an interesting situation at work that no we can't seem to figure out. A customer brought in a 1992 Lumina with Delco VI ABS in because he could not get a firm pedal no matter how much he bled the system. Apparently a brake hose had blown out and he replaced both rear hoses, calipers, pads, etc. after which he attempted to bleed the brakes. I wasn't there when he brought it in but they bled the system and still barely any pedal. This led them to believe that the master cylinder was leaking internally so a new one was installed. The master and modulator assembly were bled using the two bleeders on the modulator housing and then by the bleeders on the calipers and they obtained a good, firm pedal. One guy went to take it for a test drive and the abs light was on, as he backed it out of the stall the abs light went out and the pedal mushed to the floor at the same instant. They pulled the code (code 42-"Rear expansion spring brake hold") At this point they thought that maybe a seal had blown out in the re-manufactured I came in today as they were removing it and I took the original master apart and all the seals look great and the springs were in good condition.
I took the bottom gear cover off of the abs unit and found a piece of snap ring stuck between the threaded part the rear of one of the plungers(one of the end ones). I removed it and found that it was missing from the middle of the three plunger assemblies so I thought that maybe it got jammed in there preventing the plunger from moving thus keeping the piston down and preventing pressure from building in the system. The reason the brakes suddenly lost pressure after installing the master was presumed to be caused by the snap ring moving around and jamming randomly. Well, with the piece of snap ring removed, the assembly was installed, bled and tested only to find that the problem still exists. Someone called the used parts lot and we installed a complete used assembly consisting of the master and abs unit ready to bolt in place. This was bled, installed and bled again after which the pedal still was mushy. This should have eliminated the master, abs unit, brake hoses/lines(good amount of fluid came out of each with the brakes applied and the bleeder opened), and calipers(I clamped each line off one at a time with the engine running and tested the pedal thinking that maybe a caliper was leaking when the pedal was pushed and the fluid as well as air was sucked back in when the pedal was released but no positive results). I thought that there was a chance that the push rod got damaged but no one ever touched it and that wouldn't cause a mushy pedal anyway it should be a low firm pedal. I did some further research into the procedure for bleeding the system and found two methods:
1.Use the Tech 1 scan tool or similar to control the plungers(we don't have it so forget that)
2.Open the front bleeder on the modulator and bleed the modulator, then bleed the front brakes beginning with the right one. When a firm pedal is achieved, take the car for a drive at 3mph during which the computer should position the plungers where they are supposed to be. Pull the car in and bleed the rear brakes.
We got as far as bleeding but couldn't get any further because the brakes were too mushy using method 2. This is where we are at an no one has any ideas.
The only thing I can think of is that the Tech I scan tool is necessary to bleed the system and that is why neither us nor the customer is able to properly bleed the system. Every where we bleed the system no air ever comes out so somewhere in between there is air trapped and we can't seem to find out where.
Answer: Yes, it does sound like there is still air in the modulator valve assembly. Assuming the latest unit was never disassembled, then the pistons should be in the proper location. If the gears were removed from the bottom of the motor pack, then the valves would need to be reset. With all of the removing, replacing, etc. there must be air in the system. It is most likely caught up in the modulator assembly. A scan tool would be the best thing to get a hold of. This re-homes the valves, then the system can be bled normally. If the valves are not re-homed, air stays inside the unit.
