Just a thought, tell me if I'm wrong but
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If your rear wheels lock up then no matter what it says on your paperwork, you have NO wheel antilock brakes. Isn"t AL supposed to stop them from locking up? Maybe your AL isn't working and since the oem aren't strong enough to do the job you don't know you have a problem.

 

My thoughts were along the same lines as yours VADSLRAM

 

Even with ABS, the rears lock up breifly, then the ABS unlocks them. That's better than skidding around, but increases stopping distance over having them never locking in the first place.

I opened up the stock wheel cylinder and it has 15/16 cast into it. I went from smallest to biggest. Maybe I need the middle, but I'll wait to see how it acts in the rain.

 

thanks for posting this... I'll give it a shot asap and hopefully report back. 96 dodge 3500 4x4 with 265/75/16's

 

Well, the abs is definately working. You can hear it kick on every single time lockup occurs. There has to be something in the system that's causing it. What it is, I have no idea, only theories.

I did notice someone mentioned about brake fluid leaking on the pads. I can certainly say in my case, it has not.

I tried to think how is the best way to describe what it feels like prior to locking up, since I don't think I have accurately described what it's like. Also, I only recently resolved how to describe it, so, maybe there may be a clue for those who are more knowledgable about the braking system that could point toward the problem. So, here's my best description of what it feels like prior to lockup. Keep in mind the most noticable lockups occur at times when you're feathering the brake to a stop, like pulling up to a gas pump, or innoying stop and go traffic at a stop sign and your 5 cars back...times when your not standing on the brakes. It feels as if when you apply pressure to the brake pedal, there's almost a lag between applying pressure and when actual braking begins...it's incredibly minimal, but, noticable. Then, as your trying to feather the brakes to a stop, suddenly, you feel the rears lock and you'd swear the front calipers didn't even try to do any of the braking. It's as if the rear brakes activate first, then the fronts, and by the time the fronts start, it's too late and lockup has already happened. I thought about the lag and wondered if it's a result of the rear shoes constantly needing to be adjusted since the self adjuster doesn't keep up with the shoe wear and the lag is the time it takes for the shoes to make contact with the drum. I've also wondered if there is something that is restricting fluid pressure to the front calipers(I'm currently favoring this one). On my truck, the front calipers, brake hoses, and lines are new. The only things that are not are the master cylinder, the combination valve, and the actual abs units. Is there a way to test these? I don't think the master cylinder is a problem since I have a firm pedal and no bleed off with constant pressure to the brake pedal. The other parts I don't know what would be the testing proceedure to determine if one of them is faulty. Any thoughts?

 

Interesting the way you describe the lockup Herc.
Looking at it from a physics standpoint it seems reasonable to conclude that the rears should come into play slower with larger cylinders than with stock.
Master cylinder is the same and puts out a fixed amount of fluid per pedal pump, larger cylinder holds more fluid and theoretically should take longer to fill and extend.

Now this leads me to think (dangerous I know) that perhaps the locking problem a few folks have is actually due to something going on with the fronts or the proportioning valve.

 

I may have just stumbled onto something that could be my problem. I was trying to find info and the part # for the combination valve when I found this on an internet search. It described what a combination valve is and, more importantly, what it actually does. It turns out the combination valve incorporates the proportioning valve and metering valve into one unit. So, it turns out a proportioning valve DOES exist on this truck, it's just part of the combination valve. The following is what they have to say about each part of the combination valve. Credit for this goes to Piratejack.net.

The Metering Valve portion: "A metering valve or "hold off valve" is used in the disc portion of a disc/drum system to hold off the application of the front discs slightly allowing the slower reacting rears to catch up. This provides rear stability on wet surfaces and reduces excessive pad wear."

The Proportioning Valve portion: "A proportioning valve is used in the rear to decrease the rate of pressure rise to the drums relative to the pedal force as weight is shifted to the front during braking. This prevents the rear from locking up under hard braking conditions."

They mention symptoms of a bad metering valve is the vehicle will nose dive and the pads will wear too fast. The symptoms of a bad proportioning valve is the vehicles rear wheels will lock up easily, especially on wet surfaces.

I'm thinking these symptoms were more easily experienced when using the GM cylinders, but, are masked by the oem's.

So, this gives me a place to start. I don't think the unit is servicable, so, I'd have to replace it is as a whole. I'm sure it carries a rediculously high price. I'm wondering if there is some way to clean it without compromising any internal seals and if there is some way to verify this is the problem part. I may try to clean it at my brother-in-laws shop in his parts cleaning bin and quickly blow off the cleaner. I've done that successfully with other parts that have rubber o-rings with great success (the key is to not use anything like carb cleaner). It's looking like this may be the problem part, but, I'm not going to declare that as an absolute yet.


UPDATE: I just spoke with the Dodge Dealer and they don't refer to the part as the combination valve. They are calling it the proportioning valve, eventhough their shop manuals call it a combination valve. As long as it's the same part, that's all that matters. They told me I can look at their picture of it on their computer before I order one if I decide I need to. Anyways, they quoted me $159 for the unit. What I would like to know is if these require some type of set up proceedure or adjustment like some of the old GM ones. I'm thinking it's probably just a bolt on part, but, would like to know just in case.

 

I think not but it probably requires a special bleeding procedure. Should be explained in the factory service manual.

 

Interestingly, the only thing I remember seeing in reference to bleeding the combination valve in the service manual was it's order in the brake system that it gets bled. The last time I bled it, I started with bleeding the inlet lines then finished with the outlets and moved on to the next part. Outside of that, I don't see anything that requires special attention. That's partly why I was concerned if there is something that I'm not readily seeing. Anyways, I'm hoping I can verify this is the faulty part without having to replace it. The symptoms appear to match, but, I'd like to know for certain. The worst part is I don't have the time to do it right now and it will have to wait until things slow up for me. Until then, I'll just leave the oem wheel cylinders on. I'll let you know what happens when I get into it, just don't expect it for a few weeks.

 

My rear brakes will lock up if they are not adjusted properly. It will happen exactly as you explained Herc at low speeds feathering the brake pedal.

 

Mine does the same thing. Worse in cooler weather and almost eliminated when I keep the rears up manually.

 

Here is an article on Brake Valve operation and testing that might give you some information.

http://www.babcox.com/editorial/bf/bf50014.htm

You might also want to check your friction surfaces, I have had different brands of pads I put on the front that would not lock up even with both feet on the pedal.

Most of the new "Dustless" pads will not stop and the Lifetime ones are so hard they don't stop either.

The only ones I find work well are the Semi-metallic or full metallic of yesteryear.

It could be that your rears are working as they should but the fronts are not.
Jim

 

A fact no one has mentioned. Rear wheel antilock, ABS, RWAL or whaterver the name, all have one feature as I recall. All of them disable themselves at some low speed. Generally the "offical" speed is reported as something less than 5 MPH. I have seen many that disable themselves as high as 15 MPH. Guess what happens when abs disables itself? Wheels can lock up. If you have a marginal speed sensor it could happen at even higher speeds. Generally you cannot see this function without a factory test tool (DRBIII).
QUOTE from 2000 FSM: "The antilock brake system (ABS) is an electronically operated, all wheel brake constrol system. 2500 and 3500 vehicles have Electronic Brake Distribution(EBD) designed into the system which eliminates the combination/proportioning valve."
My non physics inclination is even though cylinders are larger, the difference in required fluid displacement is small (with reasonablly well adjusted brakes) and unable to note difference during pedal application. So the larger cyls are setting you up for rear lockup below the disable speed. In addition with abs operating above disable speed, you should not get true lockup. When speed sensor reports a wheel speed declining at rate greater the 30%(approx)(as opposed to the others) it applies pressure to offending wheel cyl(s). As soon as speed of offending wheel is equalized, the abs releases pressure and wheel speed is sampled again for excessive slowdown. A problem occurs on rears with one sensor and one control circuit for both. The sensor in differential reports the average speed of both rear wheels. So if one wheel locks and the other continues that could get the tone wheel turning twice as fast(remember diff action with one wheel locked) and mess the ABS controllers program. Oh, yes, guess why TRAC-LOC became necessary?? RWAL operates somewhat different on slowdown of rear wheels based on a alogarithm. JMHO

 

i have two 96s one is 2500 4x4 build date 7/96 and the other is a 3500 2x4 dually build date 3/96 now at the risk of sounding dumb where exactly is this load level lever i have looked on both trucks and cannot find it does anyone have pic of where it is all i have going to the brakes is the e-brake cable and the steel lines or does my model not have this feature thanks

 

To keep stirring the pot how are your flex lines? If there is to much flex in the front they might not be giving you full pressure on the fronts. Does this change work better on vacume boosted boosters vs. hydro boosted versions?
Like Jim said the pads could have a big effect on the brakes for grabbing.I would rather have brake dust and stop than not stopping and wearing the rotors away due to hard pads.

Wayne