Just another theory, but you could have a clogged return line, the smaller hose hooked up to the carb. Pull it off and see if you can blow air back through it.
It still should not leak there, but that might be part of the problem. I would expect it to pour out the overflow tubes that hang out over each of the barrels of the carb if it was pressurizing the bowl. The needle valve should hold to. Sounds like you need the new gasket for the bowl cover plate either way, but you might check out some of the other parts that could be related.
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95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)
MEPR: Man, my 4x4 makes all other LUVs look good
