If you have a Workhorse chassis, there is a little white micro switch on the upped end of the parking brake pedal that can cause problems. They get corrosion and don't allow the circuit to complete.
You can test it with a jumper wire with two alligator clips on either end. Put one clip on the small bolt head that hold the switch to the pedal and the other clip to a good ground. If you then get the jacks and slide to work, the switch is bad.
Replacement switch runs about $8 and takes about two minutes to replace.
Best Regards!