My 92' econolinea/c blows thru the defrost and floor while accelerating, I was told by a mechanic friend that it was probably the vacuum check valve, I went and bought one from Ford however I cannot find it under the hood, I traced all the vacuum hoses comingout of the manifold source, no luck. Theentire a/c system was replacedby the former owner, is it possible the check valve is built into the vacuum reservoir? Thanks