The law of physics. I pulled my NV4500, the bellhousing and the entire hydraulic system from a 1993 C30. I left the master, line and slave together so bleeding them was never needed when I installed them on the truck in 2004.
The clutch pedal felt funny back in August as I was driving to a Scout meeting. I checked and the fluid was low. I added some and it worked great again. I never saw any drops of evidence of where it was leaking. Last Tuesday I got in for the drive to work and it wouldn't go into gear. I pumped the pedal twice and it went into gear. I drove on not using the clutch for the rest of the drive figuring it was low and that I would fill it up once I got there.
I checked after work and it was full. I tried the pedal and it worked perfectly. Hmmm....? Drove home thinking I needed to find the leak and maye bleed it. No problems. The next day was the exact same thing. Bad in the morning and good in the afternoon. We were in the 50's in the am and 80's in the pm.
I have a spare used master and slave that I pulled apart figuring to just swap them out. Those plastic bores and pistons convinced me I might as well just buy new since honing isn't an option. I bought new master and slave.
I figured this was a great time to swap over to BFS, so I pulled the slave off first and left the open line over a pan. Nothing came out. I worked the pedal and nothing moved through the line. The master was still full. I crawled under the dash to undo the linkage and noticed the piston was about 1/2" from being all the way back in the bore. I tried by pushing, but could not make it come all the way back.
That means I had two problems. A leak at the slave which drained my fluid and a stuck master piston that by not going back far enough never uncovered the hole that allows fluid to drain into the cylinder. I don't know how it was still working.
I pulled the parts, blew out the line with air, put on the new master and slave and then put alcohol in the master to let the line get fully flushed out by opening the bleeder and just letting it run out. Once it was all gone, I topped it off with BFS and let it do the same until it didn't smell like alcohol any more.
My pressure bleeder was packed away in the back of the Whistler all prepped to make a road trip to see Sermis the next day, so I didn't want to dig it out. The slave cylinders don't have real bleeder nipples on them which makes vacuum bleeding not possible. That meant I had to wait until Colton was awake to bleed the old fasioned way. I got him to pumping once we got back from seing Sermis Saturday. It took forever to get any pedal at all and then even longer to get the clutch fork to actually move. We finally got it though. I pulled the M715 out of the shop and the Gasser in to change out the belts and do a few other things to it. When I went to pull the M715 back in an hour later, the clutch had no pedal and didn't work. I was able to pump it up though.
I stopped for the night discusted that something simple was getting me. I woke up Sunday with the solution though. I had never had to bleed the system and didn't think about it when I put the slave on back in 2005 with the motor swap. I put it back on the same way this weekend. With the bleeder on the bottom! Air rises and fluid goes down. We weren't getting all the air out because I had the slave on upside down. Flipped it over, opened the bleeder and a bunch of air came out. Good, solid pedal on the first try. 12 hours later it was still solid.
The moral here is no matter how you took it off, think about the simple stuff before you put something back on.