our 1969 case 580b hydraulics quit working while trenching. HELP

We are not familiar with where to start with diagnosing. The previous owners let things run down quite a bit, but we felt the price was right for our needs so bought it traveling through two states to pick it up. Back to the problem at hand: We checked to see that the input shaft on the front mounted pump was turning: IT WAS. Hyd fluid was low so topped it off! Don't know where to start with checking out the pressure from the pump. With the engine running I pulled the plug on the top of the...

our 1969 case 580b hydraulics quit working while trenching. HELP

our 1969 case 580b hydraulics quit working while trenching. HELP