Yep, that sucks. My old chair, given to me by a man at work has a normal "van" style seat on it. When I sat in it for 8-12 hours a day, it was really making my butt hurt at the end. I know what you are going through. My new chair has a ROHO cushion on it, and while I know some people love them and some hate them, it works for me. It did take me a couple of days fiddling with the pressure at first, and I kept the pump handy. There just isn't much meat left back there, and it seems I'm sitting on my pelvic bone and tailbone. I can really tell this if I have to sit in another chair for any length of time. I hope they get your old chair fixed, or maybe take the seat off it and put on this one.
Perry