The old original retainers are broken or shrunk, and don't hold the power piston in place as they should. This new split retainer solves that
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When I first tried to install it it seemed to large and I mangled it a little. After a couple of days of letting my frustrations subside I took a single edged razor and trimmed it in the split so it could squeeze smaller. I also trimmed off some mangled parts and then it worked perfectly. My suggestion is to order an extra one or two. They are cheap enough and it is better to have it on hand than to have to wait a couple of days for new ones to arrive. Don't rush it. The things actually work well once in place.
Finally! For everybody who has fought to keep the power piston down while installing the airhorn and/or gasket. It does take some effort to get it to seat in the main body but take your time and work it in.