gun leaks when I blow through it

Right I've stripped my gun for the first time tonight to clean it. I did pretty much everything by the book and it's back together looking cleaner than before.

I don't have any CO2 to test it though, I left my bottles in the office to take to get refilled on Wednesday.

Anyway with the lack of gas I decided to blow up the gas inlet hole on the handle to see if it leaked or not and it does!
(I know it sounds stupid blowing in, but I guessed if it leaked with me blowing it will do with gas).

Anyway, if I blow through the gas inlet on the handle it comes out of the barrel. Surely this shouldn't happen? I would expect it to stop me blowing through and be sealed in some way?

Anyone got any ideas what I could have done wrong or how to fix it?

I've not yet dry fired any oil through it, could that be it?

Thanks

Yes I did cock it first.

I found the problem. Took it all apart again to discover that I had the O ring in the valve on the wrong side of the tombstone. Swapped it back, oiled it and slotted the tombstone back in to place.

Just put the A5 back together again and tried the 'blow test' again and it's definately stopping it now.

Stupid mistake really, but it was my first time stripping and rebuilding an A5. I can't believe I spent so long worrying about it to be honest, it's easy.

Anyway, thanks for the help.
(it's 02:50 am over here in the UK so I'm off to bed, good night)

But when I fixed the O ring and then blew through it, I physically couldn't blow as it was stopping me.