SUCCESS!!!
I drilled a hole in each of the detent caps (1/16", a little off center, towards the bottom).
Results, when viewed from the driver's seat: from the left one a little black oil ( about a drop came out); from the right one a few drops of water came out.
When I lowered the bucket (I had it fully raised to allow easy access), a few drops of the black oil came out of the left one and a "gyser" of water came out of the right one when I pushed it to float!
Seems that amount of water could easily have been freezing and causing the loss of the float function during sub-freezing weather.
A few learnings from this episode:
1. buy a good drill bit. The first ones I bought were from the local parts store and they didn't even complete the first hole. DeWalt bits went right through.
2. Looking on-line, it is suggested to drill metal at slow speeds. So, I put away my DeWalt electric drill (3000 rpm) and got out my battery powered, slow speed drill: much better results!
I don't know if it was the better bits or the slower speed, but the result was day and night.
Thanks to all who contributed advice. This site is great!
Now I can't wait for Winter so see if it solved my problem. Ooops! I really can wait. I hate Winter.