I've noticed a decreased pumping output this week, so I made a mental note to replace the membranes on my PISA. I decided to google how often people tended to replace them and was surprised to find people talking about replacing the valves as well. Do y'all replace the yellow valves? If so how often and why? It seems like since those are hard plastic that as long as there's no visible damage they'd be fine. Am I missing something?