The latest Airgun World, Jan 2019, just arrived. It contains an article on servicing a TX200. I enjoy such articles and was surprised to see that it mentioned an Air Arms Service Kit for the TX ... Master Service Kit MSK3. I had never heard of such a kit but for those wishing to service their rifle it seems to contain everything they need... mainspring, bearings, piston seal, breech o rings, spring guide, top hat / weight and lubrication oil/grease. All for a very reasonable price. I assume the same kit works for the Prosport as well. The kit is an official Air Arms product.
The article does give a source of the kits, (Julian Bond, AirArmspares) but you seem to have to know that the kits exist or you won't find them as there is no mention in the parts list that they are for the TX (and presumably the PS as well). But careful thought could lead you to the correct conclusion as the kit is offered in a few guises as Mk1/2 or 3, presumably referring to the Mk of the rifle.
I began to wonder if AA are missing a trick in not letting it be more widely known that these kits are available, especially as they are such good value.
Cheers, Phil
PS (no not ProSport) I have no connection with AA other than owning a couple of their products.