Thanks Crunchmeister! That really helped.
I managed to dismantle almost all of it and found the problem : the part of the piston that gets pushed back when you pump broke and pieces of plastic stuck the mechanism.
I also found a replacement piston for a good price. All seems fine.
Problem is : I can't reach the piston. I can't seem to remove the pistol grip. I took of all the screws I can see but I don't know if I should pull hard, in which direction, etc... Without removing the pistol grip, I can't split open the "gear box".
This is the gun I have :
http://www.crw-airsoft.com/m1014-sho...56dl-850-p.asp
If anybody has an idea...