Mountain Biking

I’ve got some old Shimano SLX brakes from around 2002. I’m having problems with them in that the pistons aren’t reaching all the way out to the pads. I understand that after a bit of pumping they should sort or reset to near where the pads are, like a car does, but they aren’t doing it. I’m assuming the old seals are seized up or something. In order to fix this do I need to replace the inner o-rings or can I just replace the seals on the outside? I can find the seals for £5 but to replace the o-rings I need to buy the pistons and it’s more like £25.