I have an old metal 880..no diamonds on forearms. I think i bought it in the mid 80s. It would only hold one pump. I bought the new plunger and chamber seal kit. I then disassembled and meticulously cleaned the gun and carefully reassembled using a downloaded manual. When complete, it would not build any pressure at all. During disassembly, i found the pump plunger was shot, but the chamber seals looked good. Since the brass tube had some corrosion when cleaned it (where the plumger came to rest), i thought maybe it was blowing by after cleaning with a soft bristle 12ga bore brush. Since that brass tube is hard to find, i was about to toss it as a loss. I slept on it and in the morning i took it apart again down to the pump subassembly and slowly pumped and checked for leaks. The o ring on the chamber seal (the new one) was blowing by badly. I took the pump tube out and compared the old o ring to the new. It the new one was smaller OD and ID. I found one in my old Parker kit that matched the original. After reassembly, everything is fine. The new seal was clearly the problem.