Help with a Crosman 1400 please.

[Novagun]

My old Crosman  1400 pumper would take only two pumps and then the valve would operate and dump the partial charge. I have dismantled most of  it and all seems well inside. Nothing obviously broken.
My conclusion is that the spring in the compression tube behind the billet/slug, or whatever it is called, has weakened. That is the spring loaded thing that slides inside the compression tube and which the trigger sear allows to move aft to open the valve due the the compressed air in the brass reservoir.
The end cap that holds it in doesn't want to move. There is a threaded nipple on the underside of the tube that looks to be pressed in but maybe not. I am not sure if that locks the end cap in.
How do I get the end cap out of the tube to replace the spring? 
Advise please before I damage anything inadvertently.

[mav72]

Have you looked at the exploded view..?

http://www.crosman.com/pdf/manuals/crosman/1400/C1400-EVP.pdf

http://www.crosman.com/cs/manuals/crosman-rifles

[Novagun]

Thank you mav72. Got it out just needed a heavier stroke with the hammer to drive out the pin. But of oily rust inside so a clean up will help.

[dennis-baker]

The problem is the seals. All of the seals in the gun should be replaced. You need a seal kit. Please visit us in the vendor gate and we can ship you a seal kit.  Dennis Baker, Baker Airguns

[Big Bore Bart]

    Nova:   The spring is probably fine, check the trigger adjustment screws.   The trigger on these guns holds the valve closed, if the engagement is not enough, the gun will dump as the pressure rises.    This is the same reason the trigger pull rises with the number of pumps.     While you have it apart, check the trigger sear surfaces, as they have been known to chip, which causes the same symptoms.