1998 GMC Sonoma fuel system

I am rebuilding this engine, we got it back in and noticed something missing on the end of the fuel rail. It is on the opposite end of where the fuel comes in. There is what appears to be a valve of some sorts that appears to be controled by vacuum, it is attached to the end of the rail about 2-3 inches from the end. The valve is still attached, but there should be something attached to the end of it, it looks like there was a hose or cap or something that should go on the end of it, because when we turn the key on to pressurize the fuel system fuel comes out of the end of it.

There isn't any hose to connect to it. I can't find any reference to it in any manual.

Please help.
Bob