when the upstairs toilet shuts off after flushing, the pipes knock. I have read about water hammer arrestors, but can't find any in my house (built in 1976). shutting of the main valve, opening the lowest and highest valves and letting it drain, then closing everything up, solves the problem for about 2 flushes and then it starts again. Any ideas?
yes, you already seem to know the answer. By putting a hammer arrestor as close to the shut off as possible it will most likely eliminate the hammering. A tee going up with about one and a half feet of capped pipe on the feed of the toilet should give enough air gap to keep it quite.