True Union Valves

[CrazyFish]

I have two union valves before and after my return pump. I like them because of easy maintenance on my pump. But heres my problem, on the intake , I'm sure its the intake side, I have air coming out my returns to my tank. I have heard that this is a common problem with union valves, if its true does anyone know of a solution. Lube, vaseline, some short of silcone that wont bond. I know its my pump drawning air from the union valve. Thanks