Braking Issues

My truck has very sensitive braking on the drives, uncontrollable during slippery road conditions and can be unpredictable. The drives load up violently, locking at times. More noticeably is the final drive. Almost nothing occurs between 4 psi-15 psi then, the brakes violently hit the drums.

All mechanical braking components are new, brakes and push rods adjusted. Parts thus far are as follows; New S-cams, pads, drums, slacks, chambers and wheel seals as well, and no oil contamination on pads or drums, new quick release valve (QRC) located under the catwalk drivers side.

The new mechanical components made noticeable improvements, however the sensitive locking issue remains.

My thoughts and questions;

When a truck is equipped with only a single R-12 bendix valve, how are the parking brake springs governed? Is it via the tractor protection valve (TPV)? If so, can a faulty (TPV) cause dual braking (brake compounding) activating both service and parking brake springs, simultaneously loading the slack adjusters with a greater force than applied by the driver following the treadle valve? My last drive has chambers piggybacked with parking brakes, following the yellow air line may lead to the (TPV) or possibly a (R-14).

This week, I began with picking up a new treadle valve, the top also ("plunger/piston"?) rubber boot, that mounts under the foot pedal and a new R-12 valve. I haven't installed them yet, but the more I think on it, leads me to believe I may need to begin with the (TPV). Unless I failed to locate a R-14 bendix valve on my truck, the next in line would be the (TPV).

The (R-14) is a separate valve that controls parking brake spring air activated via (TPV), and used to prevent brake spring compounding.

Ruling out possibilities, this happens bobtail or loaded, so a faulty service brake check valve (prevents trailer air from compounding with service brakes) would not be the cause.

So far, no anti-lock (ABS) light, tractor or trailer. Two diagnoses at the dealership yeilding no answers. The technician claimed nothing is wrong with the (ABS) system since the abs light(s) hasn't luminated.

I have read forums describing these exact symptoms. In one case it was the treadle valve, the other (TPV), but the last did not come to a conclusion, however, commentators were ruling the possibility of a faulty R-12.

Any help would be greatly appreciated, thank you for your consideration.
1999 K.W. W900L

Any information is greatly appreciated, thank you for the video posting.

 

Thank you! Yes, the issue occurs bobtailing and also pulling a trailer. Seeing no changes are affected loaded or unloaded, I have ruled out the possibility of a faulty check valve.

Thank you for the mfg info one (r-12) or the other (r-14).

Dealing with compound, air psi or spring, one or the other. I'll keep you all posted when the install is complete, hoping by the 27th.

 

Yes, I was the valve tag is stamped with the crack psi. I read also crack psi can be affected by a contaminated valve housing cylinder ect, possibly affecting the stroke of the diaphragm to open at different psi. It could be anything at this point, hoping to rule out building replacement the valve body r12.

 

Sorry, no video, not sure how I managed to slip that in the end there.

Yes, I'm not sure on truck about different linings, however I am running Eaton brakes shoes, and they take some time to break into the curvature of the drums to seat properly.

Thanks for the reply.