I agree with the others...your problem is hydraulics. Mine did the same thing after removing and reinstalling the engine, before it was properly bled.

If air in lines is your problem you need to try to figure out how it is getting in (low reservoir?, bad supply hose?). If you correct that then all you need to do is bleed.

I tried several bleed methods unsuccessfully. The way I finally got it bled was below. You need to make sure that the slave bleed nipple is not able to suck in air with the below method (I sealed with teflon tape). You also need a hand vacuum pump.
1. Empty reservoir below master cylinder inlet.
2. Open the bleed on slave, attach tubing to the slave nipple, run into a jar of fluid.
3. Unhook the hydraulic discharge line at the master cylinder (fluid will run out of master cylinder and supply hose).
4. Hook up hand vacuum pump to disconnected tubing and pull fluid through the pipe with hand vacuum pump.
5. Close the slave bleed valve and disconnect pump.
6. Fill reservoir with fluid and allow the fluid to run through the master cylinder (messy).
7. Reconnect the tubing to the mastercylinder.
8. Hook up vacuum pump to slave, open bleed, and pull fluid and remaining air through slave cylinder.

The Haynes manual gives a pressure bleed method using the front right brake to flow fluid up through slave and master cylinder, but my front right brake bleeder was plugged (a problem I will have to correct later). If you don't have too much air in the system you may be able to bleed by doing only step 8, but it sounds like you have a lot of air in there, if the clutch stays down.

Good luck.

__________________
1976 240D
1987 560SL
2007 E320 Bluetec
1998 C280 (now son’s car)
1982 240D Manual - Sold