Looking for some advice -

I got .177 for my son to practice...sometimes, it stay on the shelf for 2 months. The air still okay, don't even leak any.
I sold the .177, and got another Marauder in .25 also, and I never have problems with air. It's a months old now. 
I guess u got one of the bad worm sometimes.
Good luck to you.
Vu

It can happen with any brand, but some more than others...

Even with a good gun shipping damage sometimes happens or it could have a faulty valve.
My air force had a leaking gauge at the fitting side a little liqued pipe sealer and it was sealed
When you are dealing with pressures like we do it doesn't take a lot to get a leak started
See what happens with the replacement

@Rlyle531 - "This post" might be useful to you. If you scroll down a bit on the first page, I compared the Streamline to the AA-S510. I think both rifles would be a great choice. 
No rifle should leak but, sometimes things happen that are not really any one persons fault. If you're returning for repair/replace, I think the M-rod is a nice solid rifle. If your returning for exchange, both of the rifles you're looking at would be a step up, IMO. 
Good luck!
Tom

My marauder went back in a hurry too and was replaced with an AA510 carbine in 22. But that was 4 years ago and no modernization on AA rifles. Have had FXs also and they're more compelling. BTW, I just got a Brocock Bantam and it's slick. The streamlines were sold out.

2 for 2 with problems on new Marauders one .22 and a 25. Refurbs purchased by friends and myself NO problems, only downside was no degassing tool.

Had a Daystate MK4 leak,My R500 showed up empty,doesn't leak (wasn't what i expected though) These things use air and run on o-rings,But you should be able to play with a new one, for awhile first.I'm sorry that didnt answer your Q, My first pcp was fx400 unregulated,But i got to hold it first.Ive had good luck/and lots of fun with the marauder line of guns.though.in the end ,I,d get the one that gives you wood when you look at it

I bought a marauder 25, shot great and no leaks. Lent it to a buddy and he kept it so long and shot it so much that he bought me a new one. The new one shot great but leaked air, about 100 psi per night. I did not want to pay for return shipping so I ordered orings to fix it myself. Turns out that it was an oring on the gauge mount that had partially rolled over on itself while it was being slid into the air tube. After replacing the orings I have not had any air leak. It was a learning experience and now I know much more about oring sizes and materials than I ever wanted to know.

My buddy later bought his girlfriend a marauder 25 and that one had no leaks. I guess sometimes a bad one gets past QC.

My MRod also had a leak. It was an easy fix since it was at the fill port. Just degassed cleaned the threads really well with some brake cleaner and used yellow thread tape that's made for gas lines. No more leaky.....

Ed