If there is a cable from inside the cab to a Y-shaped second cable going to the rear hubs? If so, there's a threaded adjusting nut on the forward cable end. Loosen that nut several turns to be absolutely sure the linkage inside the drums isn't being held partially applied.

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!