Brake Troublshooting

tgwise · February 1, 2011

1996 Chevy 3500 7.4 dual rear wheels. I have changed front pads and rear brake shoes, calipers, hoses, and master cylinder. Also power steering pump and hoses. The problem I have is the rear brakes are not braking. When on ice and brake applied the rear wheels are spinning. A mechanic that done the brake work thinks possibly the Kelsey-Hayes ABS unit is not working properly. The ABS light is not on but the brake light is on. Amost all of the brake hardware has been replaced. Is there anyway to test the ABS unit to see if is operating correctly or any way to by pass it. These ABS units are expensive, so just trying to save time and money. Thanks for any help.

I forgot to add. The brake pedal does go down a little further than normal even after it has been bled after recent brake work.

Doug_Scott · February 1, 2011

Typically the brake warning light on the dash will be on for one of two reasons, the parking brake is not fully release, of the pressure differential valve/switch is off centre (usually due to leak in system or air). The pressure differential valve simply sits between the two brake "circuits", and if there is less pressure on one of the circuits, the spool valve will be moved off centre, and usually connects the wire to ground, turning on the light on the dash. If the abs light is not on, and the fuse is still good and installed, I would think you have not bled the rear brakes completely, and there is some air in there somewhere. I do not know the procedure to bleed ABS, luckily my back gave out before that stuff showed up.

ChevyTech77 · February 1, 2011

It needs a proportioning valve. It is replaceable seperate from the bpmv (hydraulic unit for the abs). Seen it a few times before on the older KH systems like that.

tgwise · February 2, 2011

When on ice and brake pedal applied the rear wheels are turning. As stated, I have had professional brake work done and it still will not hold the rear wheels. The mechanic suggested that maybe the ABS unit is faulty. Is there anything else that could be at fault such as a check valve or something similiar. Thanks

hmoore01 · February 2, 2011

When on ice and brake pedal applied the rear wheels are turning. As stated, I have had professional brake work done and it still will not hold the rear wheels. The mechanic suggested that maybe the ABS unit is faulty. Is there anything else that could be at fault such as a check valve or something similiar. Thanks

This might sound strange however if the master cyl is not level while bleeding the system (still on truck)air can be traped in the nose of the master cyl

I found this out the hard way 1 gal of fluid and 1/2 day of bleeding system

i was on a incline driveway with the front of the truck pointing slightly upwards

i jacked up the rear of truck till master cyl was level and all the air came right out

just in case, try it

you have little to loose

good luck

Harry