Joe, That pedal to the right of the steering column(that looks like the accelerator pedal), is the air brake, brake pedal. More of the '50-'59 GMC "Big Cab" COE (Cannonballs) were sleeper cabs, than not. And most were Diesels.
Spanky Hardy Collector Of Fine Old G.M. COE Trucks & Antique Holmes Wreckers