Your lucky.....they are working like they should.
When backing hy. loads or backing up a small incline your wheels are going to spin in gravel and grass with that type brake system....was dat a trick Q'tion uh.
Mike why not let Ron P mod this section too,he's knows more than I do on things like this.add Scott to the list too,he even knows how to work on clocks,wonder if coo-coo clocks.
I've seen fender benders b/c someone forgot to put the little lever back in braking order,in a hurry you or your employee's will forget it,,may never happen to you but it could...odds it will if unlocking and locking.