I run a float valve on my sump, the float valve is fed by a largish bucket reservoir (food grade 50gal drum). I manually fill the reservoir as needed, about every 2 weeks or so.
Previously I did have the RO/DI directly up to the float valve (ie., no reservoir). I went through 4 membranes in a year. You could tell they were shot because the TDS on the post-RO stage was >100. The constant on/off/on/off cycling every 5 minutes was (I'm convinced) the culprit. Since switching to a reservoir I'm on the same RO membrane now for 3 years and my TDS is still 0 on the post-RO stage.
YMMV I guess, but personally I recommend against just hooking up the RO/DI to the sump. A bucket is not hard to come up with, it's cheap, it's effective, and it protects your investment. KISS principle all the way.
