I think the one I was working on was a straight 100, no dash, so I don"t know what yours has in it as far as the electronics for the pump controls. That being said, can you go into the computer and change the function associated with the aux hydraulics? Reason I ask is that many newer machines have the ability to make those kinds of changes, and the change will control the way the pumps react to the aux function being used, be it with the flow provided to the attachment, and possibly even the pressure.
If yours doesn"t have that capability, and even if it does actually, you can check the pressure going to the aux circuit by simply putting a guage on one of the lines and them pressurizing it. You can tee into the line and bottom out the cylinder to pressurize that particular line, or simply deadhead the line with the guage, either way will work and not cause any damage to the machine. Typical pressures used with an excavator hydraulic system are in the 4500 to 5000 psi range,...give or take a few hundred psi, but typically on the give side..., so be sure to use at least a 6000 psi guage. With that reading you can then make a determination as to whether the pressure going to the function is low, or possibly the piston seal in the cylinder is shot and bypassing.
Beyond that, I can"t really offer up much more help without the manual. While it may sound like a lot, $150 for a service manual, even one they burn on a CD, unfortunately to say, isn"t that bad of a price nowdays...given that I"ve seen them go for well over $1000.
Good luck, and while I"m pretty much out of info at the moment, feel free to ask any other questions you may have and I"ll do my best to answer if I know, or find out an answer for you if I don"t. Wayne