Hi and welcome to FixYa, I am Kelly.
Your Portor Cable C2002 was produced in 3 versions: Type 0, Type 2 and Type 3.
I will use the Type 2 version as my reference.
http://www.ereplacementparts.com/images/Porter_Cable/C2002_Type_2.pdf
There are 2 reasons for not building up pressure.
1. The connecting rod seal and sleeve: (Cylinder kit) ($19.51)
http://www.ereplacementparts.com/cylinder-kit-p-62830.html
2. The valve plate is either dirty of has failed Item 106 of the above Type 2 link PN: Z-A08548
and if it looses pressure when off ....
3. Check valve (Item 11 top link)
Try cleaning the valve plate up first. Usually when I make this repair I change the valve plate and install a Cylinder Kit. I have done this repair 4 times.
Thanks for choosing FixYa,
Kelly