The flow restrictor basically creates back-pressure. It is a bit like sticking your thumb over a hose pipe. If the water is just flowing quite steady, and you stick your thumb over it, it creates a back pressure, so much that the water squirts out with force. It is the same reason why we try to use small jets in our brushes, to create some extra pressure by restricting the flow.
long winded babble coming up...
The way an Osmosis works through a membrane is clean water moves over to dirty water. When it does this it has the same effect as if higher pressure had been placed in the clean water side. In other words the clean side reduces and the dirty water side increases. Thats basically the osmosis effect.
Reverse Osmosis is the method of basically doing the opposite. Naturally the clean water wants to move over to the dirty side (osmosis) but we stop it and reverse it, by applying pressure on the dirty side. Thus making the amount of dirty water reduce and the amount of clean water increase.
The flow restrictors job is to increase the pressure on the dirty side by creating back pressure, effectively by reducing the flow stopping the water escaping (like your thumb on a hose)
There is a limit to which back pressure can be used, if back pressure is too high it can put too much pressure on the membranes and cause them to warp. This warping reduces the life of the membrane. In saying that, there is at least one person on this forum I believe who increases his back pressure with a tap on the waste of a 100 GPD system closed up a bit to get a 50/50 clean/waste ratio and he does not seem to have/mind the reduced life span of a membrane, I would probably go that route if I had 100 GPD too, at least unless it created problems with warping.
PS, I would expect the pressure after the pre filters to increase if they were changed. Quite simply more water can now flow through the sediment filter than it could when it was blocked up.
