It's good to know that it used to be strong and now it isn't. Judging by the original poster's info, it sounds like we are looking for something that only shows up as being weak when trying to extend the lift arm cylinders. We ought to be able to find an easy fix for that.
It's possible that it could be a leaky piston seal in one or both lift cylinders. There are internal seals there and they do get old. That would only show up when trying to lift something. A quick test for that is to swap the hoses from bucket curl to lift cyinders.
A leak in the control valve itself would show up as the lift arms slowly falling down. The OP didn't mention that, and there aren't any replaceable seals in the control valve. So that's probably not it.
Another possibility - less likely, but real - is that most lever type control valves have a separate relief valve for both the lift and the curl circuits. So the lift circuit relief valve could be not working right. The relief valves can be swapped, but swapping the hoses above automatically swaps relief valves too.
To look at system pressure, I made up a quick connect "T" fitting with a 5000 psi pressure gauge. Cost about $50.00 to make using parts from TSC and the Ace hardware store. The gauge can be inserted into any quick disconnect line and tells you the pressure in that line. Handy for diagnosing.
good luck,
rScotty