On this Hyster H 50 XL after operating the lift for about 30 minutes continuously, the brake drums get very hot & the brakes lockup. After allowing them to cool the lift can continue for a short while then will lock up again. This only happens when using to unload a full semi. Normal warehouse use does not cause them to lock up' I am wondering if there is brake fluid accumulating in the brake system that is not bleeding back to the master cylinder due to the higher number of times the brakes are applied during the unloading process. And what in the brake system could cause such a restriction? There is lots of pedal & the brakes stop it fine until they lock up then they stop it too well.