How much air (pressure) is in my rifle?

Guys please help. 

I have a Brocock Contour Elite S6 that shoots very well. Yet it has no built in pressure gauge so how can I tell how much air (pressure) is in the tank?

Is there an after market gauge that I can hook up to the rifle's male foster fitting that will indicate the amount of pressure still in the gun?

Also, I have the Benjamin Marauder with the built in pressure gauge. I can never get the Marauder PSI to remain at 3,000 PSI as soon as I disconnect it from my carbon tank it drops to 2,000.

NOTE:

I had a guy who tunes PCP's (that for now will remain nameless) that cobbled together an assembly consisting of a male foster fitting connected to a pressure gauge and a sealed off foster fitting on the other side. Of course no way to connect that device to the rifle so I purchased a female foster fitting so this assembly can attach to the rifle. But the gauge doesn't move a bit.
 
Somethings not right. If I'm understanding it sounds like the "guy" cobbled together a dead head. It is a way to read the current pressure in a tank or a gun. If that guage doesnt read anything then A. The tank is empty. Or B. The gauge doesnt work. Im guessing the guage. As for the drop in pressure of 1000 psi Im wondering is it immediate (as in you pull off the hose and psssst all the air leaks out) or gradually over time. Guess we need a bit more info and maybe some pics?
 
If you trust your gauge on your tank, fill to the desired setting and shoot. If you know your shot count, such as four magazines, then just shoot that amount. I have several old rifles that do not have gauges and this is what I do. I would not trust a Marauder gauge. Chrony a shot string and seee what your Marauder is doing. If the shot string falls off too soon, then you may have a leak. If you are getting the same shot count, ignore the Marauder gauge and do as stated previously. 
 
The answer on the cobbled gauge is "c".... the female foster does not open the male foster so it can't see the pressure on the other side. No good way to do that. When you fill, you stop at your desired pressure on the tank gauge. You need to shoot over a chrono or at distance and count shots. When the velocity starts to drop( or shoot lower), that's the number of shots you can get on a fill. It doesn't really matter how much pressure is in the tank, only how many shots remain. No older pcp's had gauges that I can recall. We just had to keep track of how many we had shot.

Seems like there was another post recently about a similar Mrod gauge problem. Clogged foster fill filter?

Bob