Brakes lock up???

[ricerocket86]

Got a 98 E350 Ambulance dual rear wheels. Barely push the brakes and both rear wheels lock up. Already replaced shoes, springs, adjuster kits, drums, wheel cylinders, master cylinder twice, brake booster, ABS module. Still did it, so I disconnected the e-brake cables and disabled the ABS module. Still does it. What else is left?? What else could do such a thing? Im out of ideas

 

[rebel_horseman]

Heck you've covered pretty much the entire brake system. I've got to assume that you bled the brakes completely when you rebuilt everything. The only other thing that I can think of is maybe you have a bad electrical connection somewhere. You may try a continuity check on everything to see if you have a bad wire somewhere.

Hmm, here's another one...is the ambulance body still on or is it just the frame back there? If it's just the frame then that may be the culprit...the brake system was calibrated to stop that heavy brick that used to sit back there, so the clamp speed and force would be greater. Since it's not there anymore you may be locking up to easily. Just a thought.

 

[OLDBULL8]

Since you have replaced just about everything, my trouble shoot book says it could be the ABS unit or the height sensing valve (proportioning valve). Does it have the hydro-boost system?

 

[BigRigTech]

Prop valve is not working, applying the rear brakes before the front....The prop valve could be air locked too.

 

[foxman]

Just to share a thought. Had a problem like this once and after mand hours of head banging and agravation ??????? Found the hose going from frame to rear housing had separated inside and the inner part was acting like a check valve allowing fluid to the brakes but closing upon return and blocking flow. This one drove me bonkers + 1/2 before finding it. May not be your problem but just a thought to keep you from enduring the pain I had.
Foxman