Our old (I think 60's) American Standard, commercial style toilet (where there is no tank, but the flush valve is plumbed directly out of the wall) will frequently start flushing and run for several minutes before finally closing off and stopping. Once it stops, it refills just fine. It must be a sticky valve, or ring, but where? And how do I get to it? Must I replace the entire flush valve apparatus? They're like, $150.00! I'd love to dig into this thing and DIY it, I just need a place to start. Any suggestions?