If I brake hard, it feels as if the rear brakes release shortly after the pressure is applied. I'm assuming rear brakes since you can hear a clunk from the rear end and the rear of the truck raises slightly as all the stopping force is being generated from the front. There are no problems under light or moderate braking pressure, but if you really come down on it, you feel it right away. As soon as this happens, you can definitely feel the overall braking force is reduced (almost feels like a trailer pushing you forward).

The truck has a 3rd gen brake conversion on the front and the GM 1-ton wheel cylinders in the rear. I was thinking ABS initially since it's a short bed with almost no weight on the rear end. But I would expect ABS would keep trying to brake as opposed to releasing completely.

Wanted to get some ideas where to start troubleshooting instead of just throwing parts at it.

 

Can you pull ABS fuse and repeat test?

 

Several conditions can cause that problem, severely out of round drums if equipped, oil seal leaking gear oil on the brakes, excessive brake dust causing brake to grab. Anti lock valve is dumping pressure to prevent wheel lockup. Also hard spots on the drum will cause rear brakes to grab and lock up. Best to pull the drums and find out what is going on.

 

I'll have to give this a shot tonight.

It does have drum rears, so out of round is a possibility. Although I would expect this would cause intermittent braking from the rear. What usually happens is that once it releases, it's done till you re-apply the brakes.

No oil leaks or excessive brake dust. I pulled the drums initially to take a look and it was all dry and I cleaned everything while I had them apart. The drum wear looked even on the shoes contact surface.

One thought came to mind, would a bad rubber line to the rear axle cause this? Swelling up under hard braking pressure and reducing the effective force on the shoes?

 

Sounds like normal operation of the rear brake, height sensing proportioning valve, this is used to determine if the truck is loaded or not. If loaded more braking power is fed to the rear and if empty less, if you slam on the brakes the front dips and the rear comes up simulating an unloaded condition. Older trucks used to have an adjustable rod, the draw back with having full braking power all the time is the rear of the truck can easily lock up and spin you around.

I just looked it up and it has a fixed split point, however it may be possible to replace/fabricate the lower link with an adjustable one.

 

I don't believe this truck has a height sensing proportioning valve. I've looked all over in the past and I've never found one, because I had considered adjusting it in the past to get better overall performance from the rear brakes. The only reference to a proportioning valve in the FSM is that it's part of the combination valve that houses the pressure differential switch and metering valve as well.

 

In my paper version of the FSM is shows one and also states they are on all 2500 2X4 and 4X4. I seem to remember one on my 2001 2500 4x4 also, I think I remember it was hard to see though as it's up high.

 

If the ABS was dumping to counter a lockup, you should feel the solenoid chattering.

I would bet a soft line would not cause this as well.

Couple of questions, id this 2WABS or 4WABS system, and when you feel it disengage, does the pedal remain high, or does it sink?

 

I think they were phasing this out by 2001. I searched the 2001 FSM and there is no mention of proportioning valve outside of the combination valve. I also looked and there is nothing attached to the rear axle that goes to a valve in the brake lines.

No chatter from the solenoid, which I was I wasn't really thinking ABS.

This is 4 wheel ABS. The pedal doesn't really sink initially (at least not that I notice), but if you repeatedly brake hard while testing you can feel the pedal get softer.

 

Try pulling the fuse for the abs module, then if you have normal breaking you could have a bad HCU. You may want to check for codes also you need to need to go into the ABS module to read the codes


OPERATION
The pump, motor, and accumulators are combined into an assembly attached to the valve body. The accumulators store the extra fluid which had to be dumped from the brakes. This is done to prevent the wheels from locking up. The pump provides the fluid volume needed and is operated by a DC type motor. The motor is controlled by the CAB.

During normal braking, the HCU solenoid valves and pump are not activated. The master cylinder and power booster operate the same as a vehicle without an ABS brake system.

The valve body contains the solenoid valves. The valves modulate brake pressure during antilock braking and are controlled by the CAB.

The HCU provides three channel pressure control to the front and rear brakes. One channel controls the rear wheel brakes in tandem. The two remaining channels control the front wheel brakes individually.

During antilock braking, the solenoid valves are opened and closed as needed. The valves are not static. They are cycled rapidly and continuously to modulate pressure and control wheel slip and deceleration.

During antilock braking, solenoid valve pressure modulation occurs in three stages, pressure decrease, pressure hold, and pressure increase. The valves are all contained in the valve body portion of the HCU.

PRESSURE DECREASE
The inlet valve is closed and the outlet valve is opened during the pressure decrease cycle .

A pressure decrease cycle is initiated when speed sensor signals indicate high wheel slip at one or more wheels. At this point, the CAB closes the inlet to prevent the driver from further increasing the brake pressure and locking the brakes. The CAB then opens the outlet valve, which also opens the return circuit to the accumulators. Fluid pressure is allowed to bleed off (decrease) as needed to prevent wheel lock.

Once the period of high wheel slip has ended, the CAB closes the outlet valve and begins a pressure increase or hold cycle as needed.

PRESSURE HOLD
Both solenoid valves are closed in the pressure hold cycle . Fluid apply pressure in the control channel is maintained at a constant rate. The CAB maintains the hold cycle until sensor inputs indicate a pressure change is necessary.

PRESSURE INCREASE
The inlet valve is open and the outlet valve is closed during the pressure increase cycle. The pressure increase cycle is used to counteract unequal wheel speeds. This cycle controls re-application of fluid apply pressure due to changing road surfaces or wheel speed.

 

Just a quick update. I haven't had a chance to pull the ABS fuse and test, mainly because I did more testing yesterday and I was unable to get it to reliably release like it would before. In the past, I could hit the brakes and right after that feel the release. When I was trying it yesterday, it was harder to detect. Still feels 'off', but the quick release of braking effort was not felt.

Nothing has really changed except for the fact that I did a partial "flush" of the power steering pump when I replaced my steering box over the weekend. I pulled the old fluid and topped off with new before purging the air from the new box. Not sure if this played a factor with the booster or not, but I'll do more testing this weekend and re-adjust the rears to see what I can find.

 

Just to close the loop on this one, the brakes seem to be behaving better and I can't replicate the symptoms I had before. The only thing that has changed is that I've replaced the power steering fluid 2 more times since the box swap to get the fluid cleaned up. Guess I'll keep an eye on it with the new fluid and see if the problem comes back.

Now if I could just get the brakes to stop squealing, I'd be satisfied with all the brake upgrades.

 

Mine has always felt like the brakes let off when the ABS kicks in. Right from when I bought it. If you keep stomping on them it will stop but it does feel a little un-nerving at first.

 

Floating this one back to the top as my problem has resurfaced.

Just for updates on where it's at now:

• New drums and shoes in the rear (replaced and didn't impact the problem)
• No load sensing valve, everything is in the combination valve.
• Still has the feeling of braking power releasing. Still feels like the rear, but not entirely sure.
• When it releases, I can release and re-apply. Sometimes it comes back, but usually the pedal feel starts changing. Pedal starts to feel harder on subsequent applications.
• Pulled the ABS fuse and I was not able to recreate the behavior. Drove through the normal routes that usually feel the problems, then looped back through with the fuse in and the problem was there again. This makes me think it's going to be ABS related.

Anyone know of a good scanner that does work with the ABS computer in 2001? I've searched online, but most results seem to be cagey ("We should work, but we can't guarantee it").

 

Watch your speedo- does it register from 1-2 mph up or does it "jump" up to a certain speed like 10 mph?
If the latter, do a proper axle oil change with a cleaning of the sensor in the diff.
On a 20 year old truck I would not try to just pull the sensor from the top, the bolt likes to snap off.
If the sensor has a beard of metal you get no reliable speed signal, and the RWALB would rather have you brake moderately than spinning off the road.

In some rare cases I had a similar phenomenon due to the proportioning valve that opened too much due to the rear axle unloading.
(You wrote that you don't have one, but I think this is curious, it should sit in the driver's side frame rail slightly forward of the axle. There should be a lever connecting to a pushrod that goes to the axle. Maybe someone just wired the lever up or something like that)