Well, reading the comments I got a new radiator expansion tank cap yesterday from MB. The old one was really worn out.
No change in hose pressure.
BTW, my main concern is that it seems to be running hot as it is. The temp goes from 95 to 100 C, even on cold days at moderate speeds (no AC, doesn't work...), with plenty of air going through the radiator. It's not much of a concern now, but with winter coming I want to have the engine in as good a shape as possible.
Thanks all for your help.
1982 300D - <A HREF="http://members.xoom.com/nicodore/