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Rev JJ, the amount of weight the 70 can lift depends on how far the weight is behind the tractor & the leverage the rockshaft lift arms have on the lower lift links [assuming the correct operating pressure is present] ~About all you can do is make sure the relief valve is not set too low or you have other internal pressure problems. I presume you have the correct open front power-trol plugs in the 2 rear outlet holes now? If you didn't have the correct P/T plugs the rockshaft wouldn't even lift a little weight. ~To correctly test the P/T operating pressure you will need a 2000 psi gage, the correct remote cylinder outlet plugs, a shut-off valve capable of handling 2000-psi, a 2' or longer hydraulic hose, & some fittings a pipe "T" & some short pipe nipples.
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~You will need to install the shut-off valve on the upper remote plug with a short nipple, then a short pipe nipple, then the " T" with the gage in it, then the hydraulic hose between the gage "T" & the lower remote outlet..... Run the tractor to warm the system up, then with the lift lever in the full up position & the added valve open, slowly close the added valve & see what the pressure is when the relief valve pops open [should be 1200 psi + or - 30]. If it isn't correct you either have an internal problem, the outlet "0" rings are leaking, or will need to shim the relief valve spring. ~You might get an idea of your working pressure by just installing a 2000 psi pressure gage on the R/H lower P/T housing side plug [on the side of the P/T housing & in line with the lower P/T outlet] then trying to operate the system with a load on the 3-point. If the pressure is low it means you either aren't making the pressure or it is bleeding off somewhere. If the pressure you measure there is over 1100 psi you are probably trying to lift more than the system is designed to lift. ~You can shim the relief valve to higher than a 1230 psi blow off pressure but you risk blowing the cylinder gasket or piston ring if you do.....
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