Brake discs shouldn't need cleaning as long as they are used. If the bike is stored for a while you can clean them with brake cleaner. Brake pistons do need cleaning. If you buy new pads , the pistons will need to be pushed into their cylinders. DO NOT do this unless you have cleaned them first or you could damage the seals. Remove old pads, pump pistons OUT a few mm, I use a drop of 3 in 1 or WD40, let it soak in and then rub with an old toothbrush until all crud is gone and then polish with non abrasive stainless polish to get a bright finish. wash with aerosol brake cleaner and push the pistons all the way in , watching the level in the reservoir as you do. Fit new pads, fit to bike and pump brake until pads are in contact with discs. Ensure pads back off when you release brake lever and if not you should strip the calipers, clean and rebuild with red rubber grease applied to the seals.