Heater not working properly

Heater is inoperable replaced heater core still no heat. Lines to heater core are cold chased them to a vacuum valve on right side engine compartment it has 3 heater hoses going in top and 2 heater core hoses coming out of bottom the top ones get hot lowers don't wondering if the valve is a common problem and how and if I can test it or if its more common for the heater controls inside to go out if it is testable and how if there is anything else that could be causing my problems the valve and controller are the only things I can think of that could be causing my no heat issue and don't have a lot of cash to buy parts to throw at it. So I would really like to be able to test things before replacing them abut cant find anything on troubleshooting the problem. Any help would be greatly appreciated. I'm a truck mechanic and these darn 4 wheelers get me every time. Could it be just a vacuum problem and how can I test to be sure?