x-flow breathing/leaking

[zefarelly]

I run several cars with crossflow ford engines, why is it when you fit an aluminium rocker cover the things leak, but with a steel cover they dont.

As I understand it with a standard twin choke weber the block breather vents to the inlet manifold and this creates a slight vacuum though the vent in the rocker cover ???

with twin 40's fitted the block vents to atmosphere through a filter and the rocker cover wants to vent oil . . . . . .on one car I use a catch tank (wich can collect 150ml in a hardish 120 mile run) Im using a little K+N filter vent as an experiment on another car at the moment to see if that works without leaking . . . .

any ideas ? thanks