First thing to do when you aquire any new gun/mag is to clean the entire valve and gun. Replace any suspect orings.
When the marker is leaking out the back, does the velocity seem high? Turn down the velocity and see if it helps.
If the leak goes away, then your problem is due to the regulator piston assembly venting excess pressure. It isn't a problem unless it vents at too low a pressure such that you can't reach the desired shooting velocity.
If the leak goes away but the velocity is too low, then your regulator piston assembly is too weak for your setup. This usually happens with older regulator piston assemblies when used with higher pressure level 10 volt systems. If your mag is running a level 10 bolt and still has the original regulator piston assembly, then this is probably the source of the leak.
If the leak persists and the velocity is still high after you turn down the velocity adjuster, then your regulator seat oring is dirty or bad.
If the velocity adjusts normally, the leak out the back seems to always be there, then the oring around the regulator piston assembly is probably bad.
If the twist lock barrel rotates 360 degrees, then the twist lock assembly isn't working properly. Either it is missing, damaged, the tension is set too weak, or the grooves in the barrel are bad.
Except for the Automag in front, its usually the man behind the equipment that counts.