Be very careful removing the ring you can crack or break the airhorn as it's very thin in that area.
I use a small thin screwdriver and gently pry them out. You may have to grind one to a knife edge to get under it.
The accl pump is installed into the airhorn first, then the boot, then the retaining ring. I use a hole punch to seat the ring but make sure you have excellent support under it before driving the ring down as it's easy to break the airhorn in that area.
I use a couple of blocks of wood under the airhorn and gently tap the ring in place.....Cliff