When I repaired mine, I had a steel retainer not brass like others have had. What I did was file the crimp to remove the screen and holding the check valve in my vise, I put the short leg of a long allen wrench that fit in the retainer hole for leverage, and warmed the check valve with my propane torch. It didn't take much and the retainer popped right out.
Cleaned the ck valve up good, seated the new duckbill and using a dab of red Loctite, tapped the retainer back down. It has to seal tight or it won't hold pressure.
One thing I did learn though, is to apply the JB Weld to the edge of the ck valve first and press the screen down into it and leave it set. The more you mess with it the more the screen gets clogged. Becomes a PITA!
I got my duckbill valves from my local Husky shop for like $1.25 ea.