M-rod leaking

The first thing you might do is go with the simplest check, get a spray bottle and mix some dish soap and water, spray the fill tube and see if you are getting any bubbles. There is a check valve that on older guns can get corroded and not seal. Do the same for the gauge, it can get beat up and lose it's seal. Both of these are fairly easy to fix yourself.

As to manuals you can go to crossman and download the manuals which includes the diagrams and disassemble instructions, One great thing about Benjamin is they have so many proud owners who have modded them there is a huge amount of information online

Agree with Willie, these Marauders are easy to work on, I've stripped mine down many times and most likely you just need a o ring or one got pinched being put back together.

Google is your friend, this topic has been thoroughly covered. Soapy water is your friend, There are 4 primary leak locations, fill tube area, gauge area, valve seal area (would leak from breach screw location, and valve poppet (would leak from muzzle). Carefully apply a soapy water mix to those areas one by one letting gravity opposed the direction of water going into the pcp. Once found, degas and repair!

Behind the gauge there is a weeper hole.

Put the soapy water there.

At night put a ballon over the muzzle of the barrel.

If the balloon inflates, it is the o-ring or the poppet inside the valve.

Fill port, gauge, main valve (out the barrel) are the main areas.

Remember to store the rifle with air in the tube. 

A couple of months ago, the o-ring in the valve of my 1720T needed replacement.

As long as I was in there I replaced the poppet.

Iast month, the Gen 1 .25 started leaking. (Out the barrel).

I took the valve apart and replaced the poppet (and the o-ring)

The poppet looked like this:

Directly from Crossman for stock parts can't go wrong 

I recently went through this and found the fill assembly was leaking with the balloon technique.I took it apart and replaced the o-ring put it back together filled it and it was still there.I then read that the fill nipple will wear out or get a little piece of dirt in the seat and leak.Took it apart again this time heating the fill nipple so I could remove it and clean it.Did not find any thing so I cleaned it and sprayed silicon on the poppet and put it back together,still leaked.I then ordered a new fill assembly from Discos R Us and replaced the old one and now it holds air Yea! I also had a leak a while back that turned out to be the gauge port o-ring.I guess things wear out. 

i recently had a problem with the fill valve leaking. soap and water located it immediately...used teflon tape generously...fixed,..