Well, there is metal, and then there is lot's of different metals, and fibers and o-rings and seal materials. Sounds like some pressure testing may be called for. Best bet is dealer technician, or former dealer tech out on his own, provided he has TD15 experience.
Don't suppose you got any prior maintenance records with this machine when you got it? If it's a local rig, the dealer -- if they are still in business -- may still have some info available. Would give you some idea on previous PM and repairs. Dealer might have what they did, or may be able to call up what other dealers did if you provide serial #.
Best regards.