The vac 750 will not seal or pull a vaccum

The transformer is located in the furnace. You might want to check to make sure the furnace door is on correctly brcause theres a swich that will open for safety . Also some have a fuse on the solid state board.

Fuel can chemically decompose in as little as two weeks, especially if it's an ethanol blend. Try to avoid ethanol-blended fuels.

shutting off on a pressure switch. either high or low pressure. high likely is no condenser fan, low means a leak.

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You tackled a monster in changing the boots and vac lines. In doing the change the carbs lost the synchronization and balancing needed to start and run properly. You don't want to try the repair yourself. Take it to a dealer and have them do it. If they set the thing on fire it is THEIR problem. If you do it it is YOUR problem. Yeah, it is a somewhat dangerous process since one backfire can light things up when the air filters are not in place. Besides, they have the equipment and know-how to do a proper Sync / balance. Most owners don't. Please rate my answer. Thanks!

The "fan blades" will get clogged with a fine layer of "micro dirt" with each use. Scrubbing it gently with an old toothbrush and some 409/windex/fantastic will greatly improve the efficiency of the unit.
If it was a sudden loss of vaccum, check to make sure there is a good seal between the vac and the water canister. There is supposed to be a foam seal there on some (possibly all?) models. If that seal is missing, you will have almost zero vacuum.
Hope it's one of these fixes!

sounds to me like a jeeting issue with the carbs. If you have an aftermarket pipe then I would reccomend haveing the jetting changed in the carbs.

I experienced a similar problem with mine and I replace the little plastic tube that is immersed into the fuel. It had some stuff in it and preventing fuel from getting to the engine. It was only about 1.50 to replace.

Drained the fuel tank and rinsed it out with clean fuel then filled with fresh fuel and unit runs fine now.

dirt dust., cobwebs. where it was stored ,

I too bought my seal a meal at Walmart. I have gotten it to work twice. The main problem is it won't seal. I really think it has to do with the black seals....If I pull them up it sometimes will work. It is really frustrating. I would take it back but have mislaid my recept:(