Hydraulic Calcs

A(area)= r2Pi= 1x1x3.142=3.142 square inch
a= A-a=? have not stated rod diameter
V= volume Ax stroke= 3.142x18= 56.56 cubic inches= .92ltr

A 1 ltr Hydac bladder(availible in Australia} will serve as a tank.
Just cut the end off carefully and make an adaptor to fit the 7/8 UNF thread.

Better calcs given if you give rod size and an idea of your circuit required.

http://www.kan-opener.comwww.kan-opener.com for pics