Test the Hydrovac...
On the first start of the day push the brake pedal down and hold it, then start the engine. As the engine builds vacuum you should feel the brake pedal fade to the floor slightly...if it does, it's working. If you test just after running the engine, pump the brake 6-8 times to bleed off any stored vacuum and then re-test.
If the drums are locking up and the lines are all new, then the issue is the fluid isn't returning to the MC. You might have the pedal adjustment to tight, not letting the piston return completely or you have trash in the MC blocking the tiny port.
If you suspect the Hydrovac it applying pressure and not releasing, disconnect the vacuum line at the Hydrovac and plug it so the engine runs correctly (now the Hydrovac won't work) and see if you still have the problem. It should now take a lot more leg pressure to stop the truck so be careful!
Mike B
