If there's even a hint of slipperiness inside your 4040 housing, you have bacterial growth in there. I'd suspect that, given your description, that's the problem you have. A fresh membrane will solve the problem until it's fouled by bacteria. Flushing will do nothing as the bacteria stick like glue.
Take all the pre filters out, the carbon and the particle and your knackered membrane (throw it in the bin). Try to set your 4040 at an angle so it will be full to the brim with water when water starts to come out of the waste/pure outlets (i.e. so there isn't an airlock in there). Fill the filter vessels with water (again so no airlocks), reconnect all hoses and find yourself a big bucket. Pour 2L or 4L (it's only 19p a bottle) of thin bleach* into the bucket and top it up with water.
Dip the connector that's normally attached to your tap into the bucket. If it's too far away, run a length of hose to it, connect it and dip the other end of that into the bucket.
Put the waste and pure outlets into the bucket.
Start your pump. Run it for half an hour so the bleach is thoroughly mixed throughout the system. You should have bleachy water disappearing up the inlet hose, going through your whole system and coming out of the outlets into the bucket to go round again. You may well have to top up the bucket.
Leave for 24 hours minimum, ideally 48 hours, running the pump for ten minutes every four hours or so. You need the chlorine to be refreshed through every single part of your system.
Then reconnect everything as it should be and run clean tap water through for an hour letting the waste go into the IBC or into soil**.
Finally, put in brand new prefilters and the new RO.
You're right. This is all a massive pain, but it solves the problem. And it's nothing like as complex as it sounds. You're just trying to run bleach through every hose, connector and pump that you use for long enough that any bacteria are wiped out.
Long term, make sure there's nowhere in the hose run where dirt or air can get in - that's the root cause. I realised that I had a very very slowly dripping connector in my garden and I reckon it was drawing dirty water in when the system wasn't under pressure. And on that front, always, always, always start the tap before starting the pump. It'll help you to prevent dirt being drawn in.
Vin
*Do not, under any circumstances use thick bleach. I did once and it produced so much foam that the bottom of the garden looked like it had snowed. At the moment thin bleach is hard to find in the shops but don't be tempted. If necessary go to a cleaning supplies place to find some.
** I run the outlet of bleached water into my IBC and leave it for 12 hours then into the tank in the van for 12 hours. I know I've been shot down for this in the past but one day someone on here will kill a customer with Legionella bred in warm tank water and there'll be hell to pay. Bleach kills legionella.
