I did a brake job on a 46 ½ ton and had the same problem. I did as Scott suggested, but it still would lock up after a few miles. Only the rear drums would lock, but not the front. I re-bled the brakes and still….
I noted that I had good pressure exiting the cylinders during bleeding. I checked the freedom of the shoes, back-plate spacing, new wheel cylinders etc. So what caused the rear brakes to lock up? Turns out the brake hose over the axle was partially plugged. It would allow the brake fluid to run to the cylinders, but prohibited the fluid from returning back to the main cylinder. Turned out to be a inexpensive fix, but well worth the money. Check fluid pressure during bleeding, and note the speed of return to the main cylinder. It is a balance. Let us know what you find out.
Dale
