HW40 losing pressure - seal replacement?

[pushycheese]

Hi all
Got a HW40 which fires just fine indoors, But when I take it to the outdoor range there's not much resistance when I pull the overhead lever back, therefore when I fire I can hear a small hiss and the pellet doesn't discharge. I assume this is a faulty seal or ring which needs replacing, just thought it's strange how it works perfect at room temperature?
Thanks
Dan

[thisisdonald]

Have you been lubricating it every couple of hundred pellets? Could be dry and sticky inside the piston.

[Biker_Bob]

Put some pellgunoil on the main cylinder.
If that doesn't fix it just change the main O ring, I used one out of an assortment box I bought from a Car Boot - worked fine.

[pushycheese]

Thanks for replies, I have lubed the main cylinder so think I'll have to change O ring. Easy enough to do?

[peteswright]

Thanks for replies, I have lubed the main cylinder so think I'll have to change O ring. Easy enough to do?

http://my.tbaytel.net/coopers/HW40Review/review.html ATB. Pete.

[Stuart_B]

When I had mine I had the same problem and followed the instructions linked above. Very easy to do and the seals are pennies to buy.

[thisisdonald]

Put some pellgunoil on the main cylinder.
If that doesn't fix it just change the main O ring, I used one out of an assortment box I bought from a Car Boot - worked fine.

same here.. assorted oring job.. no problem - I use hot sauce reel oil on my webley nemesis.. because its all I have! Whats good?

[Aimstraight]

If it seems low resistance when cocking then it probably is the main seal on the piston.

However, also bear in mind the main seal on the valve itself which could be leaking. I picked up a used Gamo Compact recently that was leaking intermittently. I found the video below very useful showing how to strip the Gamo compact. The main valve arrangement is somewhat similar on the HW40 and Nemesis (but the springs arrangments differ). The problem was that the o-ring on the valve was bone dry and would not reliably seat into the valve housing; oiling the main cylinder would not get the oil into the valve quickly enough. I cleaned the o-ring, valve housing, and put some pellgun oil (no grease) on the o-rings and related springs as shown in the video below. Now the o-ring on top of the valve moves up and down and seats properly into the valve housing....no more leaks

https://www.youtube.com/watch?v=aktBRsG4q5g