Piston drag x3 advice please for a newbie

1990 Pre-cat 164 Lusso
3.0 V6
Engine Type. 6412 / Version. 164AH

Having thoroughly enjoyed the first 3 months of owning my 164 its time I spent a bit more attention to her welfare, that said, I dont want to start disturbing too much as apart from the following, the electrics are behaving, AC sort of works and the ignition, suspension and general drive-ability are still a revelation each time I drive anywhere.

Problems are, last week noticed slight drag from one of the brakes and occasional hardness in pedal feel which appears temporarily then returns to normal. Three of the alloys now become warm/hot to touch after a couple of miles, so pretty sure this is the calipers not returning. After consulting the archives here, the convention seems to be the Master Cylinder is the culprit, however checked the reservoir and ashamed to say the fluid level is at least 2 inches from the max mark with the sensor removed. There are no apparent fluid leaks from the MC or hard / flexy pipes. Before I start looking for a new MC is it possible that the low fluid level is the problem?
Whilst checking the MC noticed two electrical connections which are currently unused, can anyone tell me what they are for or should be plugged into. Pic below

I'd be checking whether they don't retract due to rust etc on the calipers first. Take the pads out and clean derust the sliders see if that helps.

Then I'd be checking the hanbrake cables are slack when released and that the mechanism that the cables pull on has returned home to its released position.

Then I'd be releasing the bleed screw at each caliper if they are still binding. If the binding goes when releasing the screw then it is a hydraulic issue.

Then I'd be trying to see if it relates to all or just some wheels. I don't know the piping routes but see if it can be caused by the flexi hoses being internally collapsed and not returning fluid or if the master cylinder is the culprit.

No point in just changing the master cylinder if the caliper, handbrake, flexis or whatever are faulty.