The boom circuit has a load check valve in the control valve for both the boom 1 and boom 2 spools. Its purpose is to prevent boom drift when you begin to input a boom raise input on the control lever. If you have the boom in the air then there's say 15 Mpa of pressure in the bottom of the boom cylinder. When you start to begin boom raise and the boom spools open, the pump pressure will be gradually rising. The load check valve prevents the 15 Mpa in the cylinder leaking off causing the boom to drop until the pump pressure equalizes with the cylinder pressure. The check valve seat will probably be worn or have debri holding them open. You'll need to remove them to check. Problem being if you haven't got a technical manual then they'll be difficult to find.
Thanks for the reply.
I have since had the problem fixed, and it was exactly what you said it was. I had the complete spool valve rebuilt. The good news is that all it cost me was parts. My best bud from school is the head instructor of the heavy duty mechanics course at a local university here and he took it on for me.
There is a poppet valve inside the hydraulic circuit that holds pressure as you stated above. Once I was able to get my hands on a repair manual I traced out the problem. If anyone else has the same problem there is an easy way to test if the poppet valve is the problem. The hydraulic circuit will also block the pressure when the travel circuit is put into operation. Therefore if you make the machine travel and then lift the boom, you'll find that it works as intended. Once you stop travelling you'll be back to the same problem again.
Apparently this is a common problem with these old machines.
Anyway, thanks for the input. I'm a happy camper now!