Classic/Antique Car Repair: 58 belair, vacuum line, manifold vacuum

Question
QUESTION: does the transmission vacuum modulator go bad easy on these cars..it shifts really hard into gear and when i hooked up the vacuum line to it it worked for short period and then it was back to shifting hard,,,do you know of anything else to look at or should i just replace the modulator

thanks

ANSWER: The vacuum modulator used be a pretty good profit item in the day that these cars were on the road. The modulator needs to be connected to the intake manifold vacuum all the time. It uses the intake manifold vacuum to determine the upshift points of the transmission as vacuum is high when the engine is under light throttle and light load and low when you are getting on it. When the modulator is working correctly the transmission should upshift out of low gear under light throttle around 19 miles per hour.  The modulator, if not functioning, can cause rough upshift as well. Check the modulator using a hand held vacuum pump connected to the modulator. Pump the vacuum up to about 20 inches and the vacuum should hold. If the needle drops right to zero the modulator is defective and needs to be replaced. The vacuum line could also be leaking. If the modulator tests ok then move the vacuum pump to the vacuum line where it connects to the intake manifold and retest. If the needle drops when tested here the problem is the vacuum line. If the modulator is ok then the problem could be a miss adjusted front band or internal problems in the transmission.

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QUESTION: does the vacuum line hook up on the backside of the carb.

there's also a tee on that line that goes to the fuel pump too,,,is this the correct vacuum line

Answer
Yep, it is the right one. A defective diaphragm in the fuel pump can also cause low vacuum at the modulator. You have the dual diaphragm fuel pump. One side of the pump supplies fuel to the carburetor and the other side is a vacuum booster to help the windshield wipers work when the engine is under load and vacuum is low.