Gas dump help

[patch]

Hello
My mate gave me a HK gas pistol he has never got it to work
You can cock&fire it dry no problem but when you put gas in the mag it dumps it out the ejection port the mag seems leak free so what in the gun would couse this I have no idear when it comes to gas so any advice would be appreciated.
Also I have noted that it needs a good grease so could that be the problem?? Many thanks
Chris