Why is coolant bleeding so difficult at times. ('03 with 3.0 six cylinder)

[cn90]

I have an E39 5-series (1998 528i) and E53 X5, basically same tricks:

1. Engine COLD.

2. Open the bleed screw near thermostat.

3. Fill the reservoir above MAX by about 1-2 inches, the key thing is PATIENCE! Sit and wait 2-3 minutes, you will see coolant coming of the bleeder screw hole near thermostat.

4. Now close the bleeder screw (gentle, no need for force).

5. Re-install the reservoir cap.

Test drive around the blocks for a good 1/2h. Let the engine cool down. Check coolant level 1h later when engine is COLD.