yeah, you don't really want to piston to come out the full way. I haven't done the rear pads yet so haven't looked at them, but sometimes the rear pistons are slotted and have to be turned to retract back into the housing, sometimes they just slide so you can use a G Clamp.
So take them off, and inspect the slide pins, pads and piston. Then make sure you empty out some brake fluid from the reservoir, just down to the minimum line, before you try to get the piston back in. If it has groves, it will spin- someone should jump in and confirm if it does. So turn it in a little way- this will force fluid back into the reservoir. Check the level, and add new fluid if needed. re-assemble, start car and let it idle, then pump up the brake pedal and make sure the piston comes back out and clamps, and you have effective braking- at least then you know that end is good.
Its a bit funny that the fluid is right up the the neck. Usually fluid drops as the pads are being worn, and the pistons push further out.