Can't remember reading of anyone yet that's used the Crosman XL rams in a 135 but maybe one to look into. Piston seals, either Air Rifle Headquarters, Vortek or Custom Air Seals are where most tend to go for better replacements and they all oversize them a bit so whichever one you end up choosing will need sized some for a good fit. The stock Hatsan seals tend to be undersized and/or nicked in a lot of cases and may be one reason for the bit lower power you're getting.You might also try Ebay. There is/was a seller on there making replacement parts for the Hatsan guns that may sell a gas ram for the 135.
It is counter productive to put another gas ram in it. The vortex are good rams and can be rebuilt and filled with air. An all around superior ram. Besides, the slight low power is likely due to seal. The gun shoots consistent now so you say so go with it until it starts to deteriorate then tune the gun. Air rifle headquarters is a great place to get the things you need for tune kit. His moly paste is the best and all you need for lube. Seals are oversized so you can make best fit and he use to have, email to ask where in catalog they are, piston buttons. You stick the buttons around the ram so it doesn't move out of center to rub one side of seal everytime you cock the gun. That's the sound you're hearing when you cock it, seal rubbing due to ram torqued to one side. The buttons stop that for longer seal life. The double size moly is far more than you'd ever need. A tiny dab will do you, less than a gram is need for one tune. [/url]