I think you are leaving the filter change to long mick, I use the 20" pre filters and used to change them every 3 to 6 months, that's when my membrane expired quickly, after reading some posts by Alex I realised that the carbon filter is only good for a max 5 weeks with the quantity of water I put through it. Now I change them every 4 weeks.
I am currently using a vyair (hf4 equivalent) membrane 99% rejection rate, it's slower than than the hf5 I had - I haven't fully checked the production rate yet, approx 250 ltrs p/h
The hf5 was belting it out at 400 ltrs p/h, but could only produce 11 TDS
Darran