installing new water valve

[jginsrq]

I am in the process of replacing a faulty water valve. it appears that I simply push both the ice maker line and the door dispenser water line into the valve. I have done this but in one of the lines there is a very slight drip coming from where the line goes in to the valve. should I have made a fresh cut in the line? what purpose does the plastic cap which sits in each of the holes where the lines fit in? thanks.