Bleeding ABS unit?

[dfitz16]

1996 Chevy pick up, rear brake line broke (rusted), replaced the entire line, can't get more than a dribble from either wheel, same at hose connection on axle. Have good pressure from M/C to ABS unit, but nothing but the same dribble from ABS unit to rear line. Do you really need a special tool to cycle the ABS motor to allow bleeding?

[900HP]

You need a scan tool to activate the abs pump. Gravity bleed won't work.

[900HP]

Bleeding Sequence:

•Master Cylinder, If Serviced
•Right Rear (1)
•Left Rear (2)
•Right Front (3)
•Left Front (4)
•Brake Pressure Modulator Valve (BPMV), as equipped (5)

Bleed Procedure:

Aftermarket Approach
•When using pressure equipment, use a special tool to depress the metering plunger on the combination valve.
•Gravity and vacuum bleeding are not recommended for this system.
•With ABS, Brake Pressure Modulator Valve (BPMV) or Hydraulic Control Unit (HCU) have been replaced or are suspected of having trapped air bleed it as follows:
1.Bleed the wheel brakes using Bleeding Sequence shown above.
2.Firmly apply the brake pedal and perform three (1993) or four (1996-2000) function tests with the Tech 1 scan tool.
3.Re-bleed the wheel brakes using Bleeding Sequence shown above.
•Inspect brake pedal feel, repeat procedures as needed. needed.

Special/OEM Procedures
•When using pressure equipment, use a special tool to depress the metering plunger on the combination valve.
•Gravity and vacuum bleeding are not recommended for this system.
•With ABS, Brake Pressure Modulator Valve (BPMV) or Hydraulic Control Unit (HCU) have been replaced or are suspected of having trapped air bleed it as follows:
1.Bleed the wheel brakes using Bleeding Sequence shown above.
2.Firmly apply the brake pedal and perform three (1993) or four (1996-2000) function tests with the Tech 1 scan tool.
3.Re-bleed the wheel brakes using Bleeding Sequence shown above.
•Inspect brake pedal feel, repeat procedures as needed. needed.

[bobalattie]

I just go out and do a few abs stops with vehicle and then repeat the normal bleeding procedure.. I'm sure you could do it on jack stands or a lift or what ever you have.. It's worked for me in the past.

[roc]

Doesn't reverse bleeding overcome this?

[Keith Morganstein]

The abs has shut off the rear brakes. Jack the rear wheels off the ground, run it in drive over 5mph and then you can bleed the rear brakes.

[turdwilly]

It's not the ABS unit. Unless air was introduced into the ABS unit during an anti-lock event (skid), there is no air in the unit & it does not need to be bled. In fact, in normal brake operation & bleeding, the ABS unit is isolated from the system so it is impossible to introduce air into it (even if you opened the bleeders at all 4 wheels & drained the system dry) unless you have a scan tool to activate the isolation & dump valves.

Your issue is that the spool in your combination valve moved to block fluid flow to the rear brakes when the line failed resulting in a pressure drop. You need to re-center the spool & your fluid flow will be restored.

[FC-Pilot]

Did you ever get this bleed? Enquiring minds want to know.

(I happen to work for a company that does brake testing for the auto manufacturers as well as aftermarket companies. I am not very familiar with that system as we are working on what we will see tomorrow, but if need be I can give you a call and we can see what we can come up with).

Paul

[Keith Morganstein]

The abs has shut off the rear brakes. Jack the rear wheels off the ground, run it in drive over 5mph and then you can bleed the rear brakes.

Regardless of nomenclature....

If you blew a brake line on one of these truck and can't get it to bleed,( dribble comes out at best)

Do what I wrote above. It works and no need to over complicate things or hire someone with a scan tool!

[rfoll]

I ran into this recently. ABS on a car a few years older than your truck. Rear cylinder leaked and allowed air to enter ABS unit. Had it bled professionally, changed master cylinder, same result as yours. GM tech told me once you get air into the ABS box, you can never get it out. Now unit required, big $$$. Good luck. If you find a way to fix it, I'm all ears.

[Rizzle]

Was helping someone out with a 90's gmc truck, replaced rear lines, rear brakes were functionally crap. A hard hit on the brake pedal and the rears got pressure and bleeding worked, so did the brakes again. Explanation was the combo valve was stuck, though its entirely possible something else happened.

[dfitz16]

No, we have had no luck at all, fronts work fine and truck is driveable. We have tried every possible way to bleed, i have locked up brakes on stone road more than you can imagine, also ran it on jack stands etc etc. i think we are going to wear out the pedal. Next step is diassasembly of abs unit.

[turdwilly]

No, we have had no luck at all, fronts work fine and truck is driveable. We have tried every possible way to bleed, i have locked up brakes on stone road more than you can imagine, also ran it on jack stands etc etc. i think we are going to wear out the pedal. Next step is diassasembly of abs unit.

Did you even try what I suggested with the combination valve yet? Of course now that you have induced multiple anti-lock events...if you had air in the rear brakes....it is now also in the abs unit.

[dfitz16]

Yes Turd we have and could not get spool to move. This is my fathers truck (he's retired) we don't always agree on best thing to try, However I sent him out to buy another unit so that we can "play" with it.