Well....I guess no one had any solution to my heater problem, so I will try this one.
My 04 Ranger Edge, mostly original 2WD 5 speed 3.0 6cyl bought new, with nearly 300,000 miles developed this problem a couple of years ago. I can't find the problem, and can't afford to put it in the shop.
During cold seasons (below freezing many nights), I have to add maybe a thimble full of brake fluid in the Hydraulic clutch reservoir every two or three mornings when it's being driven. Less than a small can for the entire season. But, in the warm season with no cold nights, I only have to add to the reservoir about 2 or 3 times during the entire season (6 or 7 months or so)! Clutch, pressure plate and cylinder were replaced at about 140,000 miles.
The only thing I can think of is the fluid line may be getting brittle in the cold, and allowing a leak around one of the fittings someplace. Then, if that is the problem the rubber line stays softer through the warm period. I can't find any problems in the part of the line I can see going from the reservoir to the bell housing. It is definitely not leaking at the reservoir. The truck shifts fine till the reservoir gets a little low.
Is this a symptom of a bad clutch cylinder? With my luck, I have no doubt the problem will end up being that cylinder! I am 77 now and there is no way I would try to pull that transmission.

 

I would be looking for a leak at the slave or master cylinder. When it is cold the rubber seals get harder and can leak more than when it is warm with the rubber seals being softer. The only other source of leak I can think of is the O-ring seal at the line disconnect where it goes into the transmission.