You're seat suspension has a flat nitrogen bottle and you're bitching about how the seat is junk.....wake up, of course it's hard, it has no charge in the accumulator.  What do you expect it to do?????

If you won't fix it, then air it up and quit bitching.  How long it stays up is dependant on how bad it is leaking.  Sorta like a tire, big hole, fast flat.  Small hole, slow flat.   

The early Magnums used a very similar suspension.  They will be hard if they have a flat accumulator, too.  So will the rear suspension on a Fastrac.  So will the header float on any brand of combine.  

Abiliene Machine, Sloan, and even Deere and Case have aftermarket suspensions for sale. Most all of them will fit in the 40 series well...certainly Abiline, Sloan, and Deere offer one which will.    The early 30 series are the ones which have the small well which has to be enlarged to take many of the air suspensions.   

We have a half dozen air seats on the place and none of them will hold a candle to a Hydro-cushion or the hydraulic Magnum seat when it comes to ride, assuming you maintain it.  Perhaps the later model stuff I haven't run will, but mid '90's air seats sure don't.