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Hi my name is***** will be glad to help. Check the QR valve above the axles for sticking and sludge inside. Verify if there is still air holding the brakes or if they are mechanically sticking as well. If air not releasing clean or replace the release valve. Let me know.
thanks for the update. See if there is air pressure at the service port on the brake cans still when released by cracking the line loose. If not then check the S cams for sticking in the bushings. Remove the pin from the yoke and slack adjuster and see if the S cams will turn freely or are binding. Let me know
So if the slack adjusters and S cams are moving freely and there is no air pressure holding the brakes applied they can not be applied unless you have a brake that has cammed over or other internal problems such as broken return springs etc.
The chamber is unlikely the cause. The return springs for the linings are what releases the brakes ans the S cam will turn to release tension on the linings. disconnect the yoke and clevis pin from the slack adjuster to check and see if the S cam is turning to release the brake.
If the foot brake valve is working properly and the release valves are new it is likely in the dash valve if you have already confirmed the foot brake valve is good. Let me know