Check Valve

Hey guys, just wondering where is a good place to buy a check valve from? At the moment, I only have a Harbor Freight vacuum pump. When you try to shut the pump off for a drop test, it just lets air back into the bag. I bought one of the brass check valves from veneersupplies.com. It seems to work good, but I can only pull down to 20mbars on my bag. Without, I was able to get it down to 1 or 2 mbars on my gauge. Is the check valve restrictive probably? Also, I’m using the Greisinger GDH200 for a digital vacuum gauge and the MTI hose. They’re awesome.

Would this check valve from Vaccon be a good buy?
http://www.vaccon.com/SearchResult.aspx?KeyWords=VCV