Classic/Antique Car Repair: Powerglide trannsmission, chevy malibu, manifold vacuum

Question
Hi , I have Chevy Malibu 1968 6 cyl with 2 speed powerglide transmission . My problem is with the transmission - doesn"t up shift. I drove up to 50 mph and  doesn't change to 2nd gear. Please tell me where to look for a problem  and what could posible be. Your answer will help me a lot.

Answer
The most common cause of this problem on this transmission is usually the vacuum modulator located on the back of the transmission. It reads engine intake manifold vacuum to control the up shift point. When the module goes bad it tells the transmission that you have your foot to the floor on the gas pedal and delays the upshift. It can be tested with a hand held vacuum pump like a Mighty Vac and replacement is a simple job. But needs to be done from under the car while it is on jack safety stands or a lift.

Brad

If I remember right it is the one that points straight forward on the bottom of the carburetor. It will have vacuum all the time when you check it with the engine running.