Issue: The rear brakes on my 96' Ram 2500 4x4 will lock up when coming to a fairly hard stop and overall braking performance is poor. 

Details: The brake pedal will slowly sink to the floor when held down. I replaced the master cylinder last year and everything was working fine up until the rear brakes started locking up. Once they started locking up I noticed the pedal would slowly sink when held down.

 

• There are no leaks anywhere and the reservoir stays full.
• The rear brakes are properly adjusted, the shoes are not worn or contaminated with oil (no seal leaks), and the drums are smooth with minimal wear.
• The front brake pads and rotors are in excellent condition.
• My gauge cluster is missing some bulb holders so I don't know if my ABS light would be on or not. 

 

I'm leaning towards it being an issue with the proportioning valve as it feels like the rear brakes are doing the majority of the work. Are these valves repairable or is replacement the only option?

It could be proportioning valve or it could be the rear axle dumping pressure in the ABS system but your ABS light might not be working.

How would you suggest that I go about diagnosing it?

I done away with my proportioning valve. You can go to Autozone and get the rear brake line and they will ask you with or without the valve. You will have to get one of their little short brake lines that have a male end on both ends because the new line has a female end and the line to tie into is female also. It was a great improvement in braking for my truck because I don’t think my valve was working. 

Like my little 1996 Dodge Ram 1500 is starting to do this too and mine is simple it just the moisture creating rust in the drums. The first couple of taps on the brakes its locking up just barely touching the brakes. 

 

As for @The_Hammer I would say double check that ABS light see if it is truly on. Might be fighting another issue with the ABS system. If you do have the load valve in the back disconnect the arm and tie it in the up position. (IIRC).

 

 

                                   Here some info.

 

 

Check out this thread...  This guy did some serious investigating which may help you. https://www.cumminsforum.com/forum/94-98-non-powertrain/2473791-weird-braking-issues-looking-second-brain-input.html