I have a '03 gmc Sierra 1500 w/4.3 & 5 speed w/215,000 miles. One minute driving & shifting great to super hard to shift w/basically no clutch. With clutch on the floor when i come to a stop it stalls out. If I start it in gear w/clutch fully depressed it takes off like an old truck would before a safety switches. Prior to this happening there were no warning signs such as noise or slipping. There is still plenty of fluid in the reservoir, plenty of pedal, just no clutch. I was hoping it was the slave cylinder but no leaks. My friend (mechanic) got underneath & watched as i pressed in the clutch & everything is working as it should as far as he could tell. Any ideas or is it time for a clutch? I just assumed it would give me some warning signs but it didn't. All good one minute to all bad the next. Any help would be greatly appreciated.