Cast iron Powerglides do not have a modulator, so there will be no vacuum loss there. Once the brake booster charges up on startup there will be no vacuum loss until you apply the brake. If you're not dancing a jig on the brake pedal, no problem there. Are you sure the vacuum piston is raising and letting the power valve close? There should be a slot in the carb base gasket that allows vacuum to get to the power piston- - - -some aftermarket gaskets don't have that slot. The OEM gasket has four slots, so it's idiot-proof- - - -no way to install it wrong.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!