I know the post is an old one, but I just saw it.
Here is my experience with the 420 shock from Progressive
I bought it and mounted it long time ago. As I remember, I never liked it a lot. It was hard then. Bike sat in storage since 2009, after maybe a 1000 miles of use of the shock, if that. So not much of use.
When I pulled the bike out of storage, I noticed that the oil seeped out of the shock, from the top, where the gas valve is. A. Echanic disassembled the shock and found the valve separated from the interior rubber bladder. It deteriorated. Apparently there is no way to reattach it, there are none to purchase, or nothing similar that we could find. Progressive was not helpful at all. They simply replied, this is an old unit, they don’t service the ST anymore, and good luck. I was disappointed, as I hoped to buy a replacement part, as I imagine , such bladder may be used in other shocks , or something similar.
Now the shock doesn’t work, and there is no rebound adjustment. I will look for a replacement for the next spring. And it won’t be progressive.