Help with faulty Old Crosman 1377

[Caretaker]

Hi all,
I recently acquired a phase 1 Crosman 1377.
Great old gun but it has developed a fault.
It appears to pump and dry fire but if I put a pellet in it won't fire it out the barrel (pellet hardly moves from the breech end).
Anyone have any idea what could be the fault?
Also with it being such an old gun where would be my best choice for spares / parts in the UK?
Any help / advice appreciated.
Thanks 

[Cross Pistol Packer]

If the pistol seems to pump ok,I would start with the o-ring on the bolt which seals the breech,if you put a sheet of tissue paper over breech & it gets blown off when pistol fires,shows air leaking there.
The price of a full set of seals is not mega money through,and if it were my pistol would replace the lot.
Ray.

[arkmaker †]

x2

Easiest troubleshooting and most likely your problem 

[NorthStaR]

UK based, you can get the seals from GMAC, eBay and this place seems to have an amazing selection
(although they seem to code the parts differently?!):

http://www.gunspares.co.uk/shopdisplaycategories.asp?id=24034&cat=Crosman

I've been meaning to try them for some parts.