I have acquired lots of old dirt moving equipment with old hydraulics and they usually have hydraulic ills.  You can usually find the problems in valves are o-rings and not hard parts as the oil keeps the steel in good shape but 40 year old rubber shrinks and cracks.  A good rubber o-ring assortment will get you started and a hydraulic shop can sell you seals and wipers for cylinders at reasonable prices.  They get them from the manufacturers who provide them OEM to the factories and you eliminate about a dozen middlemen marking up the products.  I can get seals, wipers, piston seals for cylinders complete for about 20% cost of going to ASCO which is Clark dealer on scrapers and loaders.  Works about the same with AGCO dealer.  Too bad hard parts are captive with dealer, but that is where the salvage yards come in.  Your leak down can well be in the hydraulic cylinders with oil passing around the seals on the piston rods where the pressure of the oil is applied to move the rod.  Take them off and disassemble and you will probably see wear.  Carry all of them to the  hydraulic shop (I use Lubbock Gasket in Lubbock, Texas... you can find one from someone who repairs hydraulic stuff) and tell them you need replacements and they should get you back in business.