I don't have a Hammer but, on my older Nelsons, The issue has been due to a miss-adjustment of the bolt to valve alignment. what is happening is that the bolt is touching the valve and slightly opening the valve. This can be due to a variety of causes like if you have an adjustable velocity bolt it could be set to high. On a couple of my worn guns the slot in the breach was worn to the point the hammer simply went back to far. You will need to look closely at how things align while pumping the gun and use a little trial and error to see what is causing the contact.
Originally Posted by Mr. Furious
You'll have a tough time finding a nicer group of people on the internet(even if our opinions all suck).
Originally Posted by Vitalsilence
I've dealt with canadians several times and I usually undercharge by 2-5 dollars cause I feel bad for them sometimes. Just my 2cents.