AC I600 hydraulics leaking

I recently purchased a 1967 AC I600 tractor. Both the front loader and box blade lose pressure. The box blade in 15 - 30 minutes, overnight for the front loader. I'm not a mechanic but like to perform mechanical tasks that do-able for the mechanically inclined. I've never worked with hydraulics before, but have confirmed the fluid levels are full. Do not see any external leaks around any of the hydraulic components. Is there a suggested recommendation of how one would confirm if the cylinders themselves need to be rebuilt or whether some other part needs to be have seals replaced.

Most likely just tired internal seals in the cylinders. I had very bad bleed down on my H3 loader. I took the cylinders in and had them resealed and the difference was far better than I could have expected. The seals simply get tired over time. Just make sure you deal with someone (or shop) that is reasonable. It cost me $300 to do both of them about 6 years ago. You have to decide your own standard of perfection...some shops will insist on making new barrels and that's when it starts getting expensive. I went for reseal only and it's fair to say I got close to 95% results. 

I may be wrong here, but I think it is a fram PH8A...

Probably hard to do a visual or other inspection for a leak with a load on the hydraulics in the back. If you wish to go the trouble of capping the hydraulic lines (3 point lift blocked up while doing the plumbing then remove the blocking) you eliminate the valve being your problem if the 3 point lift still lowers. With the right rear axle housing removed you can see the hydraulic cylinder inside but you might as well remove the whole hydraulic attachment at the back of the differential and take the cylinder out for testing. For the age of these tractors most of them need resealing if hasn't been done before...and maybe mostly because it's not convenient to do and then it's left undone. Both of my D14 tractors are a bit the same but not serious enough to do the repair.