Renewing O rings when they have to be removed and replaced, will make doubly sure the seal is maintained. The old one may seem O.K, but heat and pressure after tightening, will make them brittle and hard.
To help you order the correct quantity and type of seals, gaskets and other consumables, why not check our online 'parts list' fiche, or if your machine is not supported this way, simply browse the catalogue which will list everything you need for a successful overhaul.