I have that intermittent ABS light problem. Keeping the fluid level high and having cleaned and measured the wheel sensors have helped a lot. I've read some posts about 'cleaning' the level sensor contacts. Can I remove the sensor without draining or losing the master cylinder fluid?

Probably not. But if you empty the master cylinder (a turkey baster with a short piece of tubing works great) - and immediately refill it with fresh fluid after your work is done *WITHOUT* pressing the brake pedal, you'll be fine (i.e., no opportunity for air bubbles to make it into the master cylinder or brake lines).

Of course, if you've already done this much work, it'd be a great time to bleed the brakes anyway (which is one of the early steps I recommend whenever tackling strange brake problems).