So when I bought my 93 W250 it had been heavily modified, the RABS valve was removed, the ABS computer unplugged, and 1 3/8" GM brake cylinders installed with 3" shoes.
The rear brakes were brutally aggressive with the front disks hardly engaging and the rears locking up on hot dry asphalt, a smooth stop on asphalt was essentially the equivalent of drifting to a stop using the emergency brake, stopping on gravel, dirt, or wet, surfaces without simply skidding the rears to get the disks operating was impossible.

I searched high and low looking for an RABS valve, they are not to be had. I talked to the service tech at Cardone, asking if I could adapt a Ford or Chevy unit (easy to find as used candy wrappers) to the Dodge, he said yes but..... the differences were the plugs were different (they wouldn't pass on a pinout comparison of the different models) there were different sized orifices between the different manufacturers and between the different tonnages of trucks, and the controllers had different pulse duration.

Finally I found a used one I paid sixty bucks for, figuring I would wring it out with a multi meter and adapt the plug to a Ford unit, it had been sitting on a wrecked truck with the rear axle removed the lines had been cut and it had sat for three years with the lines open, I tested it and wonder of wonders it works!

Now when stopping on gravel the rears skid momentarily, and release, the fronts connect and bring it to a stop. a moderate stop on asphalt, pulses the brakes two or three times and brings it to a stop, and a full on panic stop at 45 mph + unlocks the rears and the fronts drag it down, locking up if I don't modulate the foot pressure.
Obviously it would be better to keep the rears in play longer, I assume that a reduction in cylinder bore size would achieve the hoped for effect, so in hopes of not testing out every single combination:

Could those of you that still have a complete (working RABS) system, that are satisfied with its performance, tell me what size brake cylinder bore you have?

 

The upgrade for the rear wheel cylinders is usually a 1.25 (1 1/8) wheel cylinder. I'm guessing your issue with the rears locking up is the larger wheel cylinders, but honestly think there is another reason for the rears locking up so easily. Did you pull the rear drums to check to make sure you do not have brake fluid leaking into the drums, this can cause excessive grabbing on the rear shoes. I have the 1.25" wheel cylinders and 3" shoes on my truck and no ABS valve and have no issues of premature lock up.

 

 

Just to pick a nit, 1-1/8" is 1.125" not 1.25".

I think the 1-1/8" cylinders and 3" shoes are standard on the Dually which is what I have.

 

All of the talk about wheel cylinder size is interesting and all, but none of it matters if the rear drums are not adjusted properly. I realized this when just the action of removing the drums altogether solved all of my braking problems. I know that this is not an option for some because of the cost, but it does kind of prove (to me anyway) just how problematic the rear drums can be. I believe that all roads lead to Rome and all rear drum brake problems lead to adjustment, as a first, KISS type target anyway...Mark

 

I believe you are exactly right about adjustment. This is why self adjusters were invented. One thing I have found on my truck is the parking brake cable on the right side (the long one) sticks and won't release properly. This keeps the self adjuster from operating. My next task is to replace ALL of the P-brake cables since I see no way to lube them and get things working properly. I don't drive the truck much so it's not a big problem yet.

Edwin

 

The drums are adjusted and lines are bled, drums don't get hot,during normal use (although they will get hot on around town driving as they are the only things working if I choose not to skid them) and are not dragging. The pedal is right at the top, and the fluid does not leak down. The drums are clean and score free. The Pbrake is used often and is in good repair.
Since installing the RABS valve they no longer get hot around town but....the RABS disconnects them early and often. A gentle hundred foot slow down for a turn from 40 mph to 20mph will pulse the RABS valve three or four cycles.
Frankly with, or without the RABS the rear brakes are simply too aggressive always grabbing first, and locking up before the disks are even moderately engaged.
I spent the day swapping the cylinders to 7/8 bore and frankly no joy, the rear brakes are still grabbing first,
Combination valve?

 

Does anyone have a part # for a 93 w250 combination valve?

 

 

Yes under the master cylinder, what used to be called the proportioning valve.
I assumed they would be vehicle specific, Disk/Drum is the only criteria?