Hello - I have a Sears 21 hp riding lawnmower when you push in the brake it starts fine but when you let out the brake it dies. As long as you hold in the brake in runs. If you engage the electronic blade switch (called the PTO switch) the engine dies - even with the brake pushed in. This problem appears to be caused by either the brake interlock switch or the PTO switch. How do you test, troubleshoot or figure out which switch it is? Thank you.

Without any schematics or diagrams, you could hook a meter, continuity tester and one at a time see if they switch, brakes, seat, anything else.
If not you may be able to adjust, if not, replace.