Sounds like you have some blowback in one of the valves, Had this happen before. Lawnmoore is right, you need a mech that can test it, you need to find it and thats the hard part. logic tells us that it should be the valve particular to the lower boom, not so. Any 2 cent O-RING thru out an open system can cause the pressure not to build to full power. Dont try and chase it, you will spend hours looking for it, the mech can tie into certain parts of the system and check pressure both ways, isolating the problem by eliminating the valves by testing, even tho the boom works from the bottom controls, the other thing it could be is your main pump is not spooling enough pressure to raise it, check and see if your pump is variable flow, if so it should have the ability to adjust, if it is a pump that is the problem, don't buy a new one, had one of mine rebuilt for 75 bucks, worked better than the new one that I bought to keep the truck running (didn't know I could rebuild them until after I bought a new one) so learn from my mistakes, hire a mech!
Often, guys who know a little bit, try and do it themselves, unless you work on these all the time, your playing with fire and your life! not something to play with. Although it is just hydraulics, the systems in these trucks can be pretty complicated, you'll bash your head against the truck before you find it and spend a bunch of money doing it, I know! been there, done that, have the scare on my forehead and a dent in the truck!
I ended up bringing ALTEC, cost me about 15 bucks for lunch! He stayed the whole day with me (the mech) went over the whole system and explained why it works the way it does, many fail safes in the system, they are usually the culprit, I spent/lost a total of 10,000, by not bringing in the mech right away, bought many parts I didn't need (to include the pump) main problem ended up being a O-RING that cost me $1.90 for a pack of 5, I still have 4!
