I've had exactly the same problem for a couple of years now. I can blow air through the port into the carb but very little vacuum is created at the port. My new depression chamber works fine with a vacuum pump and the linkage isn't binding. I've had the carb off twice and cleaned all of the passages. None of the "brains" on this site can come up with a solution to this problem. If you figure it out PLEASE post it, I'm leaving on a trip soon and will be towing a popup and sure could use that other barrel.

'97 Jeep Cherokee XJ, Country, Black/Gray Leather, 4.0L, 4WD Command-Track, AW-4 Auto, 3.55:1 Gears, PowerTrax No-Slip Locker, Hi Country Tow Brackets, Spectre Air Filter, 235X75R15 Goodyear Wranglers, 126K miles