Hi and thanks!,
Well, yes the cap is ONE of the problem (coolant not going from the refill
tank to the exp. tank) but if you are loosing coolant in the tank it's
because it is going somewhere else!!!!

You could have a slow leak (like one said) or a worse problem...

First change the cap and see!!!! (could be it!!! yes that could be your
only problem, IN and OUT pressure)

Second, have (or do your self) the following test done:
http://www3.sympatico.ca/huotx/pressure_test.htm

With this test you will find your problem for sure.

I hope it's not a head gasket or a crack in the head.

Good luck and my best regards, Ben
http://www3.sympatico.ca/huotx/engine1.htm