GM-GMC: 1991 GMC Hydraulic Clutch, hydraulic clutch, clutch system

Question
I'm hoping you have an answer for this one...I have a 1991 GMC Sierra 2500 with a 4-speed manual & hydraulic clutch.  The clutch was replaced and the tranny rebuilt about 5,000 miles ago at 72,000 on the truck.  The clutch loses pressure at the pedal frequently.  I thought maybe a leak in the hydraulic lines or mater cylinder and had it all replaced.  It used to be just when it was towing a heavy load up a Utah mountain road.  Now it is happening even when the truck is cold.  I have to pump it a lot to get it back and it loses pressure constantly while driving (or sitting).  The truck is now in the humid Pacific Northwest and I thought that maybe water or air in the system might be a problem.  How do I check and fix?  Thanks...

Answer
How about the slave cylinder inside the bellhousing?
As I understand it, this is the likely culprit.  I also understand that it is hard to bleed the hydraulic system used on these clutches, so I would start there.
You will get moisture in the hydraulic system.  Brake fluid, which is commonly used in those systems, is hydroscopic.  It will pull moisture out of the air and through any rubber or plastic hose.  To eliminate the problem, you have to completely flush the system with, and install, DOT 5 silicone-based fluid.  DOT 5 fluid costs a lot more than DOT 3 (the normal brake fluid).
Sorry that I can not tell you exactly how to bleed the clutch system, but I will tell you this:  You need to get the bleeder screw end of the slave cylinder as high as possible, either by jacking the vehicle or parking it on an inclined surface.  That way the air will go to the end of the slave cylinder and can then be vented by a bleeding operation.