ABS Kicks On?

If you were running larger tires, does that mean the truck has been lifted beyond factory specs? If it was, there is likely a height-sensing proportioning valve attached between the rear axle housing and the frame that is not adjusted correctly. Those are used on most trucks and minivans because they can have such a wide variation in loading. One size proportioning valve in the combination valve assembly by the master cylinder doesn't fit all conditions related to how much weight is in the vehicle and where it's placed.

Proportioning valves limit the amount of brake fluid pressure that goes to the rear brakes. Under hard braking, a lot of weight transfers to the front, making rear wheel lock-up a possibility. At some point, even though you push the brake pedal harder, no additional brake fluid goes to the rear.

For too-easy front brake lock-up, one thing to look at is the "metering" valve. That is also in the combination valve assembly. That's the brass block the two steel lines run into from the master cylinder. The metering valve is also called the "hold-off" valve. It is in the line going to the two front brakes. It's job is to stop brake fluid flow to the front brakes until enough pressure has built up to cause the rear shoes to move out and contact the drums, then all four brakes apply at the same time.

A small special tool used to be used by some mechanics to hold the metering valve open for gravity-bleeding, (the only method I use). If someone forgot to remove that tool, you'll have front brakes locking up too easily. Very few of us ever use that tool, and it really isn't needed for bleeding. It's just a piece of spring steel bent into a "U"-shape. It clips behind the small stem that's sticking out on the front of the combination valve. Most of the time you can see that stem move out just a little when a helper pushes on the brake pedal.

There are many other variables to consider if major repairs have been done in the past. In particular, there were some years where up to four different brake calipers were available. There were two different-size pistons and standard and low-drag versions for each piston size. If you got used calipers from a salvage yard, they may not be the same as the originals. If you swapped out the entire rear axle assembly, you may have installed one with smaller-diameter drum brakes. That reminds me, a common cause of front lock-up is rear shoes badly out-of-adjustment.

Combination valves are very carefully-calibrated for each specific model. It's very easy to get the wrong one from a salvage yard. Failures are rare, but the wrong one could be installed as part of crash damage repair.

Other variables to consider include the wrong diameter rear wheel cylinders, and the wrong replacement master cylinder. Even though they look the same, they have different diameter pistons for different applications.

The rear brakes shouldn't lock up except possibly below 15 mph when the system becomes inactive. Skidding tires have no traction. The purpose of RWAL, (rear-wheel anti-lock) is to prevent the rear of the truck from coming around and passing the front. Beyond that, fronts locking up too easily isn't an ABS problem.

After rereading your original post, it occurs to me you might be expecting more from the RWAL system than you should. Unlike most four-wheel ABS systems that buzz, click, and vibrate, you won't observe any of that with RWAL systems. The only thing you should observe is the rear wheels won't lock up no matter how light or hard you push the brake pedal. The only other thing you might observe is when the system activates to prevent skidding, the brake pedal will slowly sink slightly closer to the floor.

Four-wheel ABS systems have a pump to supply and / or store pressurized brake fluid. The system first blocks additional brake fluid flow to a brake that's slowing down faster than the other three. If the wheel doesn't pick speed back to match the others, a different valve opens to bleed brake fluid off from that wheel. Once wheel speed picks back up, the controller uses that pressurized brake fluid to reapply that brake. This sequence of "block, bleed, apply" can repeat from 15 to 30 times per second as long as it's needed. That's the buzzing and pulsing you observe with those systems.

RWAL systems don't have the reapply step. That's supplied by you with your foot on the brake pedal. When the rear wheels together slow down too quickly, your valve assembly blocks additional fluid flow to the rear. If the wheels don't pick up speed to what the computer thinks is appropriate, it opens the "dump" valve inside the valve assembly. That bleeds off brake fluid pressure to both rear wheels. Once wheel speed picks back up, the dump valve closes and the block valve opens, then you are able to push more brake fluid to the rear if you choose to. That's when the brake pedal will drop a little. You'll never hear or feel the valves operating, but they are preventing the rear wheels from slowing down too quickly. The system has no idea what is going on in the front.

Drum brakes take some time for the shoes to move out far enough to contact the drums. Four-wheel ABS systems operate too fast for most drum brakes to be effective, so you'll usually find disc brakes on the rear with 4-wheel systems. Drum brakes are much better matched to the operating characteristics of RWAL systems.

You might also observe if the easy front lock-up occurs mostly in the morning or right after the truck has been sitting. This could be due to a light film of rust build-up overnight on the rotors. I had that problem with my '80 Volare since it was new. First time in the morning the rear drum brakes would lock up and skid under very light pedal pressure at the end of my dirt driveway. After that, the rust film was worn off and the problem never occurred on any roads until the next morning. I still have that car and it still does that once each day.

Also check if there's damage to the splash shields behind the front rotors. That can allow excessive moisture to collect on the rotors. We normally associate that with one type of brake fade, but it can also cause the linings to grab too aggressively. That too should occur intermittently; not all the time.

Consider unplugging the valve assembly, then see if the rear brakes lock up. If they do not, the fronts are doing all the work. That would send us in a different direction. Normally it would be easier to pull the two fuses, but the fuse diagram shows one of the RWAL fuses is also used for the wireless communication module. That could affect something needed to make the truck run.

Okay, so we know the ABS is working correctly.

I can make a comment of value for the rears not locking up in reverse, but it only applies to drum brakes. Some lightweight cars use a "non-servo" rear drum brake in which both shoes are the same size. One applies going forward and the other applies in reverse. In the case of the older K-cars, when the front, or "leading" shoe wore out, you could switch them front-to-rear and get twice the life out of them.

All larger cars and trucks use a "duo-servo" rear drum brake. With those, the leading shoe, (toward the front of the vehicle), is smaller, and the trailing show is larger. The trailing shoe does the stopping. The leading shoe's job is to grab the spinning drum and try to rotate with it. As it does, it pushes on the star wheel adjuster assembly, and that pushes the bottom of the trailing shoe into the drum.

At the same time, the rear piston in the wheel cylinder pushes the top of the trailing shoe into the drum. So there's two forces acting on the trailing shoe. It does all the work when going forward, so it has to be bigger to last as long as the smaller leading shoe.

When you back up, the roles are reversed. The larger trailing shoe acts on the bottom of the smaller leading shoe. This time the front piston in the wheel cylinder pushes the top of the leading shoe into the drum. Since that shoe is so much smaller, it is much less-effective at stopping that wheel when backing up.

To add to the misery, when stopping when backing up, more vehicle weight is transferred to the rear, so the tires grip the road harder. That makes lock-up even less likely.

In addition, drum shoes are self-energizing, meaning once they contact the rotating drum, they tend to get pulled into it and grab harder on their own. Disc brakes don't have any of those benefits. They aren't self-energizing, so all their stopping power has to come from your foot on the brake pedal. This is why 99 percent of vehicles with disc brakes have a power brake booster.

Another test that I thought I mentioned already but looks like not here, is to stop on a slight incline, shift to "neutral", release the brakes, then see if the truck creeps downhill on its own. If it does not, suspect the front calipers are not fully releasing. On some models such as my older Caravans, that is real commonly caused by the metal bracket crimped around the middle of the front rubber flex hoses. Rust builds up inside the crimps and constricts the hose. You can easily force brake fluid under pressure through the restriction to apply the brakes, but then the fluid can't return on its own. The brake pedal will be too high and too hard, and the brakes will grab easily. Your hoses don't have that metal bracket, but we still run into a fair number of flex hoses that tear inside and form a restriction. This can happen faster if a caliper is allowed to hang by the hose during routine brake service.

If you find a brake isn't releasing, place a block about a foot downhill of a tire so you don't look funny chasing after the truck, then crawl underneath and open the bleeder screws. If that allows the brake to release, pressurized brake fluid is being trapped somewhere and is unable to return back to the reservoir. We figure out where that restriction is by opening the hydraulic system at various places.

The worst case is if a brake releases when you loosen the steel lines right at the master cylinder. That is caused by brake fluid contaminated with a petroleum product such as engine oil, transmission fluid, power steering fluid, axle grease, or penetrating oil. That contamination causes rubber parts to swell. A clue is the rubber bladder seal under the reservoir's cap will be blown up and mushy, and you won't be able to pop it back in place. In the master cylinder, the rubber lip seals grow past the fluid return ports and blocks them. Another clue is since the brakes are dragging, they get hot, and that heat migrates into the brake fluid causing it to expand. Since it can't get back to the reservoir, it applies the calipers harder and harder.

I don't think you're going to run into this contamination, but if you do, the only proper repair is to remove every part in the hydraulic system that contains rubber parts that contact the brake fluid, flush and dry the steel lines, then install all new rubber parts. That includes the master cylinder, calipers, rubber flex hoses, combination valve assembly, ABS valve assembly, and when used, wheel cylinders, ABS controller, and rear height-sensing proportioning valve. If any rubber part is not replaced, the contamination will leach out of it and recontaminate the new brake fluid.

The only exception to this is if a contaminating fluid is poured into the reservoir and the mistake is caught right away. In that case I would suck out as much fluid as possible with a turkey baster, then remove the steel lines at the master cylinder and run a full small can of clean fresh brake fluid through it. Every year I did a demonstration for my students on the effects of this contamination. With two inches of brake fluid in a small beaker, I added a wheel cylinder lip seal and one drop of oil or power steering fluid. It took a week, then we found the contaminated seal was slimy, mushy, and it had grown by about 20 percent. The key is it took a week.

After doing some research, it seems a lot of people have the same complaints of easy front brake lock-up. Quite a few conclude it's due to rust build-up on the rotors overnight and the problem only occurs once in the morning, as I described with my Volare. No one can really point to a definite defect.

What I find interesting is no one complained of this when the trucks were new. All manufacturers spend a real lot of time and resources developing brake systems that are carefully balanced front-to-rear. That involves piston diameters in the calipers, wheel cylinders, and master cylinder, drum diameter and width, square inches of pad contact, and valving. The main point of importance is when any aftermarket brake parts manufacturer offers replacement parts, they have to maintain that brake balance. It doesn't matter if they use different friction material, the combination of "coefficient of friction, and physical size have to equal what was originally designed for the vehicle. Cheaper linings might have less resistance to heat-related brake fade, for example, but under normal conditions they will perform just as well as more expensive linings.

Given that wondrous information, I wouldn't expect to find the wrong brake pads, but what about the calipers? In the '90s there were four different caliper designs that could be bolted onto one corner of the truck. Two had an 80mm piston and two had an 88mm piston. (The other variable was low drag for better fuel mileage, or normal calipers). If you were given the wrong ones at an auto parts store, those with the larger pistons would bolt right on, but require the brake pedal to move a little more than normal, and the caliper would apply significantly more pressure to the pads. The different piston sizes were meant for different versions of options on the truck, and they weren't meant to be changed to a different size.

There's one more thing I can share, but I'm a little hesitant to suggest this. There was a service bulletin related to early front pad wear on the larger trucks that were constantly heavily-loaded. We had quite a few in my city that had huge boxes with all kinds of tools for plumbing or heavy equipment. A half dozen in particular were for a company that installed and serviced hoists, and they were grossly overloaded all the time. To alleviate the fast front pad wear, the service bulletin instructed us to increase the diameter of the rear drum wheel cylinders. For these grossly-overloaded trucks, we could go up two sizes larger, but that wheel cylinder wasn't available through Chrysler. They had to come from a GM dealer. In every case it was an effective solution to the rapid pad wear on front, but that is not something we would alter for the typical daily commuter. These work trucks were so heavily-loaded, that locking brakes was the furthest thing on their minds. The modification made the rear brakes work harder, thus taking some of the load off the front pads.

I'll keep researching this to try to find you a better answer.

I really didn't suspect contaminated fluid. Just didn't want to overlook anything. Other than the rubber seal under the reservoir cap, you won't see anything else swollen unless you take something apart.

The piston diameter I described refers to single-piston calipers from the mid to late '90s. I brought that up because there's quite a few different part numbers listed for your model and year. They're all of the two-piston design. We have to trust the people behind the parts counter to sell us the right parts, and they don't always get it right.

Brake fluid normally gets dark from being hot. All manufacturers have a specified interval to change the fluid because it loves to absorb moisture from the humidity in the air, even through the porous rubber hoses. We rarely follow those recommendations because the systems are so reliable. I'm happy you did flush your system. I like to drive old stuff and the steel lines often rust out, so I'm putting in new brake fluid every few years anyway. The lesser issue has to do with the steel lines corroding from the inside. The bigger concern is moisture in the brake fluid lowers its boiling point from well over 400 degrees Fahrenheit to 212 degrees. It's real easy to reach that temperature under normal conditions, then the boiling water forms air bubbles in the fluid. That leads to a second type of brake fade.

Rather than wasting time pulling a caliper off, let me suggest visiting an auto parts first and asking them if they can see in their books anything that differentiates one caliper from another. They can be a good resource when they find information in the books they get from their suppliers.

I'm here every day. I'll be waiting to see what you find.

I can't find any reference to a height-sensing proportioning valve in the rear hydraulic system. That surprised me, but it is likely the engineers assumed the RWAL system would take care of that.

After discussing this with a few other people, we kept coming back to the metering valve in the front of the combination valve assembly. To add to the insult, I can't find any reference to this assembly for 2007 or newer models. Seems this is also handled inside the RWAL valve assembly. In case I'm wrong, here's a photo of a typical combination valve. The blue arrow is pointing to the end of the metering valve. That used to be a small stem poking out, and you could physically see it move when a helper pressed the brake pedal. This one is covered with what looks like a rubber cap. My guess is you should be able to feel the valve move under that cap when your helper pushes the pedal. If that valve is stuck open, the front brakes will apply too soon, but also remember, its purpose is to delay or hold off the application of the front disc brakes until the rear drum shoes have had time to move out to the drums. You don't have rear drum brakes, so a metering valve should not be needed.

I also found this on AllData:

DESCRIPTION - ELECTRONIC VARIABLE BRAKE PROPORTIONING

Vehicles equipped with ABS use electronic variable brake proportioning (EVBP) to balance front-to-rear braking. The EVBP is used in place of a rear proportioning valve. The EVBP system uses the ABS system to control the slip of the rear wheels in partial braking range. The braking force of the rear wheels is controlled electronically by using the inlet and outlet valves located in the integrated control unit (ICU).

But they aren't clear as to whether this is part of the RWAL system or the four-wheel-ABS system.

Another thought that came up was glazed front rotors. That results from the pads becoming so hot that the "binders" or glue that holds the linings together melts and forms a coating on the pads and on the rotors. That glazing can be somewhat sticky and cause the pads to grab too aggressively. For my own vehicles, I don't waste time trying to sand that coating off. I take a light cut on the brake lathe to leave very tiny grooves in the rotors. Those act like a file and scrape the glazing off the pads within a few miles. That has been a real effective solution to solving brake squeal complaints too.

I'm not giving up on this. I'm still looking for other suspects or solutions.

There was a service bulletin in the late '90s about going one or two sizes larger for rear drum brake wheel cylinders, but that was to address rapid front pad wear on larger trucks that were constantly heavily-loaded. Other than those types of factory-approved modifications, I don't recommend changing anything in the brake system, mainly for liability reasons. The truck didn't have easy front brake lock-up when it was new, so we have to keep looking for the cause.

Any chance the rotors have some kind of contamination on them? Another thought is to check for worn caliper mounting hardware that allows them to twist a little when the brakes are applied.