Secondary not opening (G54B), no vacuum?
I know these trucks (with the 2.6) are notorious for the secondary throttle actuator not working. That being said: YES I have checked vacuum with a pump, mine held vacuum for 15 minutes before I released the pressure. The vacuum hose is going to right nipple on the carburetor.
I have played around with vacuum lines a little bit, and if I put the secondary's vacuum line on any other port, then my secondaries open, but if I open the throttle enough to use them, my truck will idle really high (~2k rpm) and will diesel after shut off. I have played with my throttle when the secondary is connected to the proper nipple, and manually open the secondaries (while on the correct nipple), I do not encounter the high idle or diesel, it continues to run at normal idle.
This leads me to think that I am somehow not getting vacuum through the port on the carburetor? That's my only logical guess. I pulled off the valve cover wondering if the [also notorious] jet valves could've been prohibiting vacuum draw somehow, but my valves work 100% and are NOT stuck. I rotated the engine over until all jet valves had been opened and closed and they all return to full rest like they should.
You have a weird issue with the secondary not opening. Jet valves shouldn't (?) affect internal vac in the carb. Try hooking a vac gauge to the secondary port and see what kind of vac is present while the engine is running. Only things that come to mind are either there's another actuator or vent control that is sharing the same internal circuit as the secondary and is shot - or there's an internal leak due to a damaged gasket.
This thread has been sitting for a bit, any news?
Does your secondary butterfly close fully, and opens only allowed to open when the primary is nearly fully open? Rotates freely and the linkages are lined up correctly? When on another port does it always idle high, or only after engaging the secondary? Could definitely be some vacuum problems, but maybe also some mechanical hijinks.
I'd also suggest the vacuum gauge, but also maybe get a T and put the gauge on the windshield while you're driving so you can see it properly under load.