I have an HF5 4040 running inpumped at around 40 PSI. The membrane is about 3 years old. Tap input c. 200. RO output c. 008.
When I flush, no pure water is made. I have a valve by the RO output that I adjust. When flushing, it is fully open. When producing, it is slightly open (allowing about 50:50 waste:pure or maybe 60:40).
I'm no RO expert but that seems to have worked OK for about 3 years. The TDS does sometimes climb a little if I've been too tight with the valve adjustment and it does sometimes seem to start rising a bit before the pre filters are replaced.
EDIT: I say that no pure is produced when flushing but in all honesty, if there is a tiny trickle, it would be hard to spot the way I have it set up (opaque JG tubing). If it's just a trickle, I wouldn't hear it as it would probably just run down the inside of the tank slowly.
I will disconnect the pure pipe next time I flush in order to check.