If the pump keeps running you likely have a vacuum leak. If the bellows are leaking you likely will have sewage in the compartment.
While the vacuum switch is reliable they do fail. The switch is located on the accumulator tank which is why the pump wouldn't stop running with your hand on it.
If you are lucky sometimes some of the liquid will drain without a vacuum but don't count on it.
Parts are available to rebuild the pump. https://marinesan.com/
has some diagrams on their page. Fast shipping and good phone support.
Shaving cream on the fittings will show where the leaks may be. It draws into the leak.
You said: When the vacupump is running and I activate the pressure sw on the tank, pump quits.
To me that indicates problem with the vacuum switch OR you have a leak. I have manually turned the adjusting screw about 1/8 turn till I got desired results. It is not recommended so do at your own risk.
I would buy a rebuild kit that includes the bellows, gaskets and duckbill valves. You have no history of the pump, you have been into the bellows and the duckbills are the next suspect. They aren't hard to rebuild, lube the duckbills, dish soap works well, and don't over tighten. Don't overlook the slip-fit gasket that goes into the accumulator tank. Six bucks it might be worth it if you suspect it.