Tuning Simple marauder leak question

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Found a leak, super super slow, this was overnight.

Exploded parts diagram does not take the fill assembly any further down. Repair or just replace? 
Thanks in advance. 






 
De gas the gun, remove the foster fitting and examine the plunger seal. I have never had mine out of my Marauder, but they normally have an O ring under a plunger that seals them. It could just be a small piece of crud that is keeping it from sealing or it might need a new O ring that you can find at a hardware or auto parts store. PA has them for $15.99 in stock. I would order from Crosman.
 
My Marauder, when new, also had a small leak at the threads for the foster fitting. I sprayed a small amount of soapy water on it to pinpoint the exact location. I removed the foster fitting and cleaned all the old black thread tape out. I couldn't get any type of tape to stop it. Tried liquid pipe sealer too. I used locktite 545 and let it sit for a couple days. It's made for high pressure pneumatics and hydraulics. That stopped the leak.