Why do some Sheridan streaks require cocking before pumping?

[OneDollarBob]

I've resealed around 10 over the last year, and most will pump up cocked or un-cocked.
A few just burb air out unless you cock them first and I can't tell why.
I want to say it was the earlier models with this issue, but I can't be certain.
Does anyone have an explanation for this behavior?
If I had to guess, it could be that the hammer spring has been stretched out by age or some attempt to get more FPS.

[avator]

Heavy hammer spring or weak valve spring... or both.

[airgunaut]

I think the Sheridan manual says something about older models needing to be cocked before pumping.

Mine will pump without cocking first. I am not sure of its age; best I can tell is very early 1968.

Here is the text:

[Back_Roads]

 A weaker valve spring will insure full valve dumps at full power. or a heavier hammer spring will do the same but harder to cock.

[OneDollarBob]

Thank you guys.

I assumed it was something with springs, but its just counter-intuitive.
I guess cocking takes any pressure from the hammer side to allow to valve to seal better?

I've looked over that manual many times and never noticed that highlighted section and that it discourages leaving the rifle cocked for extended periods.
I store mine cocked with a few pumps of pressure.  Maybe I've been doing it wrong.

[OneDollarBob]

Nevermind. I think I read that wrong.   It states "Does No Harm" to leave cocked.

[Ronno6]

Oftimes I have been able to pull the bolt back a bit, not to cocked position,
and put in a pump or 2.
Give it a try.......

[Habanero69er]

Or, you can just fully cock the rifle, put in a few pumps. Then hold the bolt as you pull the trigger releasing the bolt slowly.
Then no springs are stored in a bind.

[JPSAXNC]

I seem to remember there was some small differences in valve stem lengths.

[19Sheridan57]

  You will see this also on Sheridans that have been “ beefed up “ a bit. I have several here that I have to do that with & these are shooting around 700fps, which is above the average 650 . As long as there is not any air retained, you can consider that a plus , with the added power.
    Added ;
     To see if your rifle is retaining air, pump it up 8 times, the maximum , load and fire. Then cock it, no pumping , & fire it UNLOADED again. You should only hear a “ thunk .”

[MDriskill]

Pretty obvious from the previous excellent responses I think, but to be perfectly clear - when not cocked, the hammer physically rests on the outlet valve stem. The valve spring often requires a bit of pressure behind it to avoid being overcome by the hammer spring.

As Habanero69er described, it's easy enuff to cock it, put in a pump or two, and then un-cock the hammer. That's what I typically do with mine.