valve problem

I am diving in a viking pro dry suit for contaminated water. All is
fine. Except I have problems with the valve. Or Should I say valves,
for I have yet to be diving with a completely safe valve (safe here in
the meaning that it does not let ANY water in). I think the problem
might be from the fact that it is mainly shallow water diving (1-7 m.).
Obviously it is not very clever to dive in contaminated waters with a
perfect suit and then have valve which simply cannot keep out water,
unless you close it completely at all times.
Are there any valves specifically made for low presure/shallow water
diving? Fitting Viking?

Mads