Just recently replaced the Clutch slave cylinder and led it with a pressure bleeder. Noticed when I pushed the clutch it was still spongy....noticed when I was adding 15psi to the system that the Fluid reservoir was leaking out of the back.....I replaced that and bled it again....def much better this time...goes into gear and drives...but if you hold the clutch down for longer that a normal shift (say you were riding the clutch in reverse) it seems to bleed away pressure.....after a pump (usually one pump) it comes back up and works fine......

Is it common for the plastic line going from the Brake reservoir to the slave cylinder to get a leak?

 

So it is def bleeding out pressure....it works fine with regular shifts (clutch in, then right back out).....if you hold it down for a few seconds, you cant shift...but one pump and it is right back up and working....also brake fluid is less after driving for a bit.....now just to find the leak.

 

You didn't bench bleed it? Take it off, clamp it compressed and fill the slave. Add a little bit at a time keeping the cylinder vertical. Keep tapping it and youll be amazed how many bubbles just keep coming. Plug, install, quick bleed and done. Just the pressure pot will not work.

 

So I had it completely compressed when I bled it with 15lbs of pressure in the system....3 times just hanging and I tilted it towards vertical...
then i took the cap off the fluid res.....topped the fluid off, ..left the fluid res cap off.,,,,,.sloooowly let the cylinder expand fully......then nestled it back into the mounts.

 

That should have worked fine man, maybe a bad part. Is it an OEM part? I had an amazon and an AutoZone cylinder both fail on me damn near instantly. Went to the dealer, no issues.