If you think that the pistons are sticking, take both wheels off, take all four pads out, keeping them in order.
Then press gently on the brake pedal, and you will sense the pressure moving the pistons. Don't overdo this or one will pop out and you'll have to bleed that side. Look at the pistons and how far they have emerged. The one furthest out is loosest, so you can retract the piston and replace that pad. Hold it in place temporarily with masking tape - they have been known to drop out during this process. Look at the remaining pistons and see which has moved least. Give it a small squirt of lubricant. I've useed WD40 for years with no ill effects - but keep it away from the discs. You could also use silicon spray lubricant. Once sprayed, retract the piston, and keep doing this to each piston until they all emerge at the same rate. Replace the pads and put a smear of coppaslip on the backs.
What's probably happening is that the pads are sticking to the discs, which is common after a wash, as mentioned above.
