86coupe

Hi, I have a question about the differential pressure regulator. The little box attached to the fuel dizzy. It was leaky on mine, and I took it off, only to discover that one of the o-rings was busted. So I got some new o-rings at work. But what was surprising to me is that on the regulator itself, there are actually trhee(!) holes, two of them match with the fuel ports on the dizzy, but there is a smaller on the side as well. Anyone know what this one is for? and do I need any kind of gasket or only the o-rings?
thanks in advance.