I did not follow all details that were discussed here.
But I want to give a general advise:
It is always a good idea to write down the behaviour of a program in
normal but precise-naming words and if numbers in any kind are involved using example-numbers (= digits 0-9)
To avoid any misunderstandings caused through misconceptions about coding this description should avoid all programming terms.
As a small example of what I mean:
Pressure tank is at 1 psi
Compressor starts running to increase pressure up to 10 psi
The Arduino measures pressure continously
if pressure reaches 10 psi the unloader-relais shall be switched ON for 15 seconds
......
Such a description gives a clear picture of what the whole code shall do. Based on this description the other users can make a well suiting suggestion to achieve the wanted behaviour (for example a state-machine based on the switch-case statement or whatever)
Focusing on details gives detail answers which might keep the overall structure of the code in a bad suited way.
best regards Stefan