There is a safety system that runs trough the brake switch and the pto switch for starting. Then there is a different safety circuit that runs through the brake switch, the pto switch and the seat switch. It is possible to have part of the brake switch bad, but more than likely the pto switch is bad or the seat switch wiring is torn up. I have worked on one 525 that the seat switch was bypassed in a way that it allowed the wire to short to ground. In doing so, it burnt up the diodes that come out of the bottom of one of the relays.