If they need a lot of pressure to work, I'd say they're most likely dirty over the carbon pads. I don't quite remember if the PS2 uses rubber domes with a contact underneath or if it uses a two layer flex circuit, but either way both can be cleaned. You could disassemble them and clean the carbon pads and under the rubber domes with some isopropyl alcohol and a cotton swab. At least that's what I do.