I was having the same problem with my JD4210H, my solution was to install an old drum brake shoe tensioning spring outboard on each side. They pull the park brake linkage back, ready for next application and don't interfere with normal brake operation. One end in the hole on the linkage, the...
I tested the parking brake, it was not set and engine dies or labors, with it set or not when PTO is engaged, I immediately shut PTO off and engine comes back to set throttle position.
I just got my rockshaft controls and rate of drop control working. They had quit functioning due to seizing up, and while working on these items I attempted to engage the main PTO, it starts turning and the engine dies at any throttle position. Seat wires jumped.