The clutch system can be tough to fill from the top. When I replaced the clutch slave cylinder on my W201 a few years ago I filled it by opening up the slave bleeder valve a half turn and connected a length of clear vinyl tubing to the brake caliper, which was full of fresh fluid following a complete flush.

Then my helper pushed on the brake pedal and I opened the caliper valve to pump fluid into the clutch slave. The clutch system shares the same fluid reservoir as the brake system, so once there were no bubbles in the reservoir, I was done and the clutch release/engagement was right on. A spray bottle with water washed off the fluid that seeped past the bleeder valve threads, and I topped off the reservoir.

I don't think it took more than about six pumps of the brake pedal to fill the clutch system. Filling it from the bottom up rapidly purges the air.

Duke