There is a seal (o-ring) under the pump that keeps the pressure contained as it flows out of the pump and into the fluid gallaries. If you have the pump out you may have lost this seal and now your pressure is bleading away. I am not very familiare with these old cable plows but I believe they are very similar to the isarmatic electric version which I know has this seal.
Apto Quod Victum "Adapt and overcome"
Western factory trained installer / mechanic.
Oh and I have a plow too... a 2005 Ford F550 with Western Wide-out