Water running back into holding tank

[Ghetto Wash]

I've never had a brass union like this not want to unscrew 20 or 30 years later.

I don't use those unions any more. I use flare unions. Much easier to use and last just as long.
https://www.amazon.com/Pipe-Nut-FLARE-SWIVEL-CONNECTOR/dp/B005JI1QYC

 

[MEP001]

I used a pair of stainless 1" JIC flares as a union once, it was about half the price of a 1" brass union that had already once eroded to leaking.