I used to think you guys were nuts, but I too recently became a "brake-on victim." I had gotten off the tractor to adjust a chain tied to a small tree I was trying to remove.
When I jumped back on the tractor, it was groaning and straining as I pulled back and forth... I couldn't believe that sapling was putting up such a fight!! When the tree was finally out of the ground completely, I noticed that the 'bota kept groaning and straining... oops.
A simple inductive proximity sensor (no moving parts, water tight) and a sunlight visible LED is all that you'd need for a very simple and rugged installation...
For $16.75, you can get a little sensor that's water tight.
Inductive Proximity Sensors Overview
Jesse