will power brakes booster work with vacuum connection in rear of carb, vic jr does not have a vacuum connection

It should work fine, it's the same as manifold vacuum which is what you need. Just don't share it with a PCV.

Will work fine if you use the one in the rear of the base plate (large one)

for those interested, i found out it does not work. when you take foot off gas throttle plate closes and you no longer have vacuum

Should have anything to do with throttle plate positioning...the fittings on the front and rear of the carb are exposed to the manifold. Sounds like your not making enough vac or your brakes have an issue.