How can I bleed the cooling system entirely?

When changing the cambelt (thanks again for all your tips!), I tried to change
the cooling/fluid, but only 2,5 liter came out. The manual says there should be
8,5 liters. I used the small tap on the bottom of the radiator. The fluid was
only a bit dirty, not really worrying; the quantity however was. I think I only
bled the radiator and reserve-tank, not the fluid in the engine. While filling
up, about the same amount (2,5 liters) went in.

Now the watertemperature seems ok (90 degrees C) and constant, but I feel the
engine gets hotter than I can remember and the oil seems thinner (oil pressure
is lower than normal). But there have been some hot days here in Holland and
this is also unusual for our climate. Than again: the engine seems to suffer
from vapourlock, after two minutes running idle at the trafficlights there's no
power and the engine shuts off. So this to my opinion is also a clue that the
engine is running hot.

Any clues? Bleeding the entire system seems like a good first. Only questions
remains: how?