Regarding the diameter hoses i set it up with some high pressure hose I had left over. I don’t know the exact dimensions but it would be smaller internal dimensions to that of a normal hose pipe.
I could try swopping that out maybe.
Put some bigger diameter hose on, your 4040 will want a high water flow (as well as pressure) and you’re restricting it. Let the pump worry about the pressure.
So regarding the pressure.
Tap pressure is 40psi.
RO BEFORE booster pump used to be 30psi
RO with booster pump ON was 40psi
Now
RO BEFORE booster pump is 20psi
RO with booster pump ON is 30psi
Now 30psi just isn’t enough pressure to maintain the membrane. So it’s sniffed it in 6 months.
The pump when working is only raising the pressure up to 40psi (so raising by 10).
That’s awful.
For context, my mains pressure is 40psi, my booster is set to 80psi (I could have it higher but that pure/waste ratio seems to be my sweetspot).
So couple of things to try.
Your waste valve isn’t set correctly so
close your gate valve a little and watch the pressure go up.
Or
Your booster pump knackered.
(Might also be worth checking your prefilters. If you’ve not changed them for a while they may be clogged up, restricting flow. Check the sediment fileter and see how “dirty” it is if you can’t remember when you last changed them.