My similar dated 77 is also doing exactly that... Max is with express (14.3 or 4grn I can't remember) pushing 10.3/4 ftlbs & 9.5-10.4 over a few different brands. Now, I stuck a nose extension in it, a new seal and put 2 of tinbums power washers in and it stayed the same. It shoots nice so I'm a bit loathed to change it as it's my pleasure gun where power don't mean jit but it is niggling my thoughts a little. Still.., I'll stick a few more tins through & see what happens I think..