End dump rear axle locking brakes up at 5 psi... REWARD OFFERED!

Good afternoon, I hope everyone is having a great weekend, and thank you in advance to anyone who is willing to take time out of their lives and busy schedules to read my post! I'm just about done pulling my hair out over this trailer. I've got a 1993 Lufkin frameless end dump with Neway trunnion-type suspension. I pull it with my 1997 FLD120 Freightliner with 3406E Cat. Shortly after buying this trailer earlier in the year, I noticed that it was becoming easy to lock the brakes up on the back axle only, somewhere in the neighborhood of 20psi empty. I noticed the shoe linings were well worn, so I had a shop rebuild all brakes, including new s-cams, bushings, new shoes, new drums, wheel bearings, wheel seals, and 30/30 round port standard brake chambers. The problem still persisted, and the brakes would lock up easier now, just on the back axle. If I backed the slack adjusters off almost as far as they would go, no brake lock-up, of course. The front axle on this trailer never locks up. Even at 50psi of brake application pressure, no lockup. Replaced relay valve, and spring brake valve. Same problem. Replaced trunnion bushing. Same problem. Replaced brake hoses, same problem. I then started with Bendix' tech team to troubleshoot my problem. They seemed to think the shop who rebuilt my brakes used wrong parts. I hunted down my original equipment information on my Dana/Spicer axles, now having to talk to Hendrickson due to them now owning Dana. I rebuilt one side of my back axle's brakes, including new shoes, drum, s-cam, and s-cam bushings. Same problem persists. I can be driving down the road with all brakes adjusted 1/4 to 1/2 turn out, 55mph, apply the brakes and the rear axle alone will lock up at anything greater than 5 psi. No kidding. It is nothing to lay 1/4 mile of rubber with this trailer just trying to slow down. I have to turn out those slack adjusters over 2.5 turns on the back axle to get it to not lock up. I'd like to use my trailer and put it to work, just having this major issue. The dealer that used to sell these Lufkin trailers took a look at it and could not figure it out, only suggested that my trunnion is having some issue and recommended replacing the trunnion tube, bushing, and all cast iron parts just to make sure it is within spec. To the tune of several thousand dollars. Anyone with the problem-solving idea will get a $50 gift card, and I'll probably make it more to express my thanks!! Has anyone had a similar issue with a center pointed trailer?

 

I should mention that today I rigged up an air hose from my framing nailer with an air gauge, and tested pressure at the gladhand and at the brake air hose coming from the ports of the relay valve on the trailer. No pressure spikes, if I gave 5 psi of foot feed pressure, the gladhand showed 5psi, and pressure from the relay valve also showed 5. All the way up to 50-60psi. I also replaced the spring brake valve, which was an SR5 and replaced it with a Sealco 110700, which has an anti-compounding feautre built in. Still no fix.......

 

Thanks for all the great advice and ideas, I was gone all weekend and finally last night was able to get back and jack this thing up in the air directly under the trunnion, which would have logically lifted both axles off the ground at almost the same rate..... I almost couldn't believe my eyes when the front axle stayed on the ground and the back lifted up....almost 2 feet in the air before the front axle lifted too. With both axles in the air I jumped onto one of the back tires and it would only spring down about an inch as I jumped up and down on it....believe me that trunnion is locked up TIGHT..... I don't have any idea what could cause a brand new polyurethane trunnion bushing to store that much torsion in it and lock up like it has, but here we are... It took a floor jack with some wood blocks under the front axle and almost all my weight on the jack handle to jack the front axle back up, it was squeaking and creaking all the way when we could get it to budge. Jacked it to where it was level with the back, test drove and wouldn't you know it.... took 10-15 psi this time to lock up the back axle. Still locks up, and I'm probably just going to trade this thing off, but it helped significantly! Not sure if I want to spend the money on a whole trunnion replacement....