Trailer Brakes Rear Axle too Sensitive

I've got an issue with a non abs spread axle flatbed Trailer where there rearmost axle brakes are pretty sensitive and locking up. They unlock as soon as I release the brake pedal. So wondering if there is some sort of proportioning valve for air brakes? I was almost thinking about using two 1/2" air pressure regulators off the service relay valve to lower application pressure to the rear axle but not sure if that's the best idea. Or possibly replacing trailer service relay valve with one with a higher crack pressure but not sure if that would help enough.

Originally I thought spring pressure was what applied the brakes and air pressure releases them but have been reading it's actually positive air pressure that releases and applies them so just also wanted to confirm that.

This is mainly happening when unloaded and thinking there is not enough weight on the rear most axle with 11r22.5 trailer tires and low pro drive tires so the suspension in the rear axle looks higher than the front.

Trailer does have new drums so not an out of round drum and s cams looked in decent shape when changing shoes. If there is anything else I should check I would appreciate any advice or opinions if I'm remotely on the right track here.

 

I tightened the automatic slack adjuster until pads were touching and backed off half a turn, but haven't measure stroke yet. Would they lock up easier with too much or too little stroke?

 

Yes this trailer has air suspension. There does appear to be some sort of a dump valve wired in between front and rear bags but all the bags still seemed to have air in them. The rear most axle air bag does seem higher so I do think there is less weight on it but still seemed aired up

 

The rear axle tires on my trailer are regrooved so front axle is probably a little taller tread. I could maybe swap the regrooved to the front axle but the airbags are probably a least 1" higher in rear higher.