Yup, the part you're looking for is H43-26, I think it's called piston cup.
All you need to get is a new piston head, this guy has 2 for sale
http://www.airsoftcanada.com/showthr...pa+piston+head
this guy has enhanced loading nozzle and piston head
http://www.airsoftcanada.com/showthr...pa+piston+head
EDIT. Daviet is right, what I meant to say was an UPGRADED piston head would fix your problem as they( most as I can tell) use an o-ring on thee piston head for a seal and not the piston cup thing