D4-7U hydraulics will not hold blade up slowly drops within a minute or two

do you have one lever or two?

Diverter valve handle should point to the right for dozer operation, straight back for rear implement operation.
Blown hydraulic ram piston seals are most likely cause of leak down.
Get a part number off the cylinders.

Only has one lever, I'll move it to the right and get the part numbers as suggested. Should I try that Lucas hydraulic booster with stop leak?

one lever. then OM suggestion would be right on. do the packings leak around the polish rods?

Well, I did not see any visible leaks after about 30 minutes of use and when raising or lowering it operates smoothly.

Well, I did not see any visible leaks after about 30 minutes of use and when raising or lowering it operates smoothly.

As old magnet says most likely cause will be an internal leak around the piston seals. I don't know what medium was used for those seals. If rubber a hydraulic stop leak may work. How that stuff works is it "attacks" the rubber causing it to swell. At best it's a temporary stopgap measure. Mechanic in a bottle stopgap measures may work for a time but are not going to last like a proper repair. They may also cause excessive wear or destruction of parts in the system.

Rick

Sounds good, I'll start researching on how and what's all involved to remove and replace the bad piston seals.

There are at least two different kinds of seals, one of them are composite rings with a stepped end gap. It is possible to use a modern piston seal and make a new groove in the middle of the piston if needed.