I found a good description of the EBC4 ABS unit bleeding procedure at www.autozone.com
Click on: Repair Info; Repair Guide; Brakes; Four Wheel Anti-Lock 4wal System 1983 - 1993 GM Blazer/Jimmy/Typhoon/Bravada. If you are bleeding by foot pressure, be paitient and don't give up becauseit takes a long time to push the air out of the ABS EHCU. It may take as many as15 to 20 separate attempts to get a firm pedal. A pressure bleeder is what you need for this type system.
I bled my EHCU according to the instructions on theAutoZone website and had good brakes for about 50 miles then they got soft and the peal would go to the floor. I am am convinced that if my 18 year old EHCU was good at the time, I would have good brakes now. That is why I pruchased a rebuilt EHCU for $700. Autozone has Cardone rebuiltEHCUadvertised but they don't have any in stock and cannot get any because Cardone is out of cores and do not expect to get any more. I purchased arebuilt Raybestos EHCU and it was one of the last 10 units they had.Rock Auto Parts recently dropped the Raybestos EHCU from their on-line inventory. Rock Auto Partsstill has the rebuilt Cardone EHCU advertised but they are not aware thatCardone is not offering them anymore. I talked to a Cardone represenative about this matter. Raybestos and Cardone are the only companies that rebuilt those EHCU.
You must usethree OTC-7853 depressor tools to bleed the system.Placing duct tape over the vales does not work. When using the depressortoolson the two low pressure accumulator valves and the rear (proportioning section) of the combination valve,DO NOT turn the wing nut down but finger tight or no more than .030" when depressing the valves.Remove the rubber boots on the two EHCU accumulator valve stems and attach the tools.The combination valve stem doesnot have a rubber boot.The proportioning valve stem on the combination valve islocated on the opposite end from the rear brake line attached to the combination valve that goes to the EHCU .The OTC 7853 depressortools are available at www.carparts.com
for $5.00 each. The same tool isas much as $30 each from other suppliers on the internet.
Those are the tools thatthe so-called "ASE Certified" technicians DO NOT have and that is why they do not know how to bleed these type ABS brakes. They do not want to pay $15 for three tools that they would never used but once every ten years or so. A technician at a large Chevroletdealership told me that! I hope that ASE Certified Technicianis laid offin a few months afterGM goes broke then hemay be able to get a jobas a fry cook at McDonalds.