I think SMH has the parts most likely, as should a Clark dealer or Clark in Louisville Kentucky. you might try searching google for "Clark Material Handling", or ask the parts department at your local (**** near any brand) forklift dealer.
I am not clear on if "you have one wheel that turns" or not, but you should be able to feel some (+/- 1/16th of an inch 'jiggle') "free-play" in the linkage/actuator shaft -at- the master cylinder when the pedal is released. It is possible to have this shaft over adjusted and not have any relief/return of fluid from wheel cylinders to master cylinder reservoir. this has the effect of having free-play in the pedal linkage, but locked down brakes.