Hayes HFX 9 eh? Ok, I'm assuming that the pistons won't even push in all the way. Sometimes the system gets pressurized. Chances are you might need a bleed as well, but here is something you can try. First, take your pads out. Next, loosen your lever and point in upwards so that little bleed plug is pointing straight up. Pull that plug out, but be careful. You don't want to rip that rubber bladder in the reservoir. Go ahead and wrap a rag around the lever and that bleed hole. If you can get a box wrench in the caliper, that's the preferred method. Use either an 8mm or a 10mm. If you damage that post or the piston, you're going to need more parts. What you need to do is slowly work those pistons back in. Slowly is the key. As you push, fluid will come out of the lever. Once you have pushed them all the way in, put the plug back in the lever. Clean all the fluid up and put your pads back in. Put the wheel in and try it out.
I have done this a bit at the shop. However there are the times that the brake needs a full bleed. Just figured its worth a shot and might save you a trip to the shop.