Detent position inside valve wears down, allowing control to slip out of float position without normally required light tug. I think the rebuild kit is about 25 bucks from Kubota. I ordered the kit, but my dealer ended up installing it for me for another 25 bucks (50 total) while my tractor was in for 1250 hour service. He showed me how it was done - it seemed simple enough.
Removing condensation from behind cap didn't work for me - she was just plain worn out. Works like new now, just like the rest of the tractor!