1998 GMC Sonoma

If anyone could help me I have a question. I have a 1998 gmc sonoma 4 cylinder 110000 miles. The problem is either my slave cylinder or the master clutch cylinder. In the summer time I have no problem my truck runs well but as soon as the nights get cold my fluid is empty in the resivoir. I fill it up and pump the clutch to build back pressure and everything is fine. But there is never no fluid on the ground. Now I fill It up it the cold mornings It will allow me to start and put in gear but after about 10 seconds it will bog down and die and there is no pressure on the clutch until I pump it up again. And about after doing this a couple of times my resivior is empty but still no fluid on the ground. Please someone help to determine what the real cause is I dont have that much money right now to replace both of them.