I am working on this same forklift. I am suppecting the brake controller to be the problem but do not know anything about how this type of system works. The problem I am having is when the brake is pressed it dramaticly bogs the engine down and will cause it to die if held there very long. The normal hydraulic does it slightly and the steering. But nothing like the brake. It only happens really bad if you are just barely engaging the brake control valve or pressing it more firm. But in between isn't as bad but still pretty boggy. And that is why I am thinking the brake controller is the problem. Any info would help me figure it out better. Thanks!

have the same problem does your truck have a brake fluid filled master or is it like mine (GC25e). that has a Hydraulic valve feed from the Hydraulic tank the same tank for the mast.

If the pedal sinks to the floor then the seal is likely gone. SMH sells kits for most of them or go to a hydraulic seal place with your samples. Last hydraulic brake valve I priced from dealer was about $500.00