Trailer brakes locking up at low application pressure....

Hello all! Got a trailer that's giving me fits... it's a 2014 Travis 39' end dump with Hendrickson Intraax suspension. Now I've had a trailer in the past give me lockup problems but that was due to a bound up trunnion bushing, this one is totally different, but behaves the same, even with a different truck. Old truck was a freightliner, new truck is a 2003 Pete 379EXHD with a C15 and air trac suspension. Trailer was used with old truck and new truck. Problem is, when I press the brakes, at about 5-8 psi the trailer just squats and feels like the trailer is dragging the truck. At 12psi or over, the trailer tires will lock, smoking the tires. Most of the time the ABS will kick on and vent pressure, but about half the time the tires will lock up and just leave 25 yards or more of rubber and I'm barely slowing down with no ABS. I've rebuilt the brakes, using materials that are the same as what came off (4707Q shoes), drums, s-cam bushing tubes, and re-torqued wheel bearings recently. Still the same problem. I've had the trailer to the Travis dealer, who said everything looked fine. Only replaced an ABS sensor they said was not providing signal. Same problem. I then took the trailer to another large dealer here in Denver (Transwest) who said that one brake chamber had a rod that was too short so they replaced that, otherwise the only problem they can find is that they say the brakes are adjusted too TIGHT. I know they are automatic slacks, but I've tried backing them off to mitigate the problem here and there, to no avail. They tell me that they replaced the brake chamber that was too short and then adjusted the brakes, approximately 1.5 turns out from the shoes hitting the drum, in order to get 1.25-1.5 inches of pushrod travel, which they say it should have. They say that this is all the trailer needed, heading over tomorrow to pick up the truck and trailer (I left both with them) to try out but my experience so far says if you have to give it 1.5 turns out from shoe hitting drum, something is wrong? Why can no one seem to be able to pinpoint the problem even after taking it to multiple shops and replacing all parts?

 

I've also plumbed a line and a gauge from the brake air line to the service side of the chamber to make sure the ABS valve isn't dumping too much pressure into the chamber, and it mirrors the applied pressure gauge in the cab exactly. I've checked for air back feeding from the glad-hands when aired up. I've double checked ride height with Hendrickson, I've troubleshot the Sealco parking brake valve with Sealco support, I've troubleshot the Wabco ABS valve with Wabco support, and everything is fine. The shop did say today they thought the ABS valve was releasing brake pressure too slowly, but didn't think the valve was bad? There's a lot of info here, I appreciate any help at all. I can't keep eating flat spotted tires, especially during the year we are having this year....

 

Thank you for the good info! I can’t express how thankful I am for this forum and for everyone willing to take a few minutes from their schedules to give feedback on these problems. I only hope my knowledge continues to expand enough to make suggestions on here enough to maybe help someone out too. I think I solved the problem though! I wound up just tearing into everything and found that the s-cam bushing tubes were shot even after just a few short months of use, and worse, on the rear axle had begun to turn in the spider, compounding the wear issue.... I put on new tubes with bushings and per your suggestion Sauer I went ahead and got some of the 20k lined shoes (abex) and once the brakes were set up I tack welded the tubes to the spider until I can either have the spiders replaced or maybe try having them reamed or bored for a larger diameter tube. This should last I am thinking till the next brake job, and I’m thinking of trying one of those kits that come with the reamer and heavier duty tubes with bronze bushings and case hardened s-cams, had anyone had any experience with these kits? Is it worth it to bore the spiders and press in heavier duty tubes? I have a shop tell me they’ve had mixed success, and just makes the tubes harder to replace when they go bad again down the line...

Brakes are back to nice, easy to feather brake applications now though!