The unit is a foodsaver V835, it's about 8 years old and I bought it new. It has seen its share of abuse. When I first started using it, I overfilled the bags that I was trying to seal, the liquid over flowed and got into the unit. This happened a few times. After learning how to use the unit, there has been no further 'accidents'.
Recently the unit was shutting off when I stopped holding down the lid. The way it should work is once you put a bag under the lid (for sealing) and you push down the lid, the unit should pull a vacuum and seal the bag without you having to hold down the lid.
Time to find the problem. First to remove the cover, the lid is held to the base by 2 pins that fit into holes in the base. Just pull the edges of the lid so you can remove the pins from the holes in the base. Caution, do not pull to aggressively or you may crack the lid. Next there are 6 screws on the bottom (4 of them are under the rubber feet). Now you can separate the upper and lower cases and expose the 'guts'.
I started by figuring out how the unit works. You can see the suction lines (plastic tubes) and they lead from a vacuum unit to various points to draw air from the bag and there is port on the top of the unit that allows you to attach a tube to pull a suction from a canning jar. Check the tubes for any blockage. One section of the tube attached to a pressure sensor. This is seen in picture #6 and it is a round red device. The sensor monitors the amount of vacuum in the line and controls the unit from turning on and off. There is an adjustment screw on the underside of the sensor where you can change the amount of pressure when the sensor will turn the unit on or off.
This was the problem on my unit. I took off the circuit board from the base and made adjustments to the sensor. Viola problem fixed. Just to avoid further problems, I took off the gaskets that seal the bag and turned them over. If the gaskets leak the vacuum, the unit will also act up.
Now all that's left is to reassemble. One note: make sure you reinstall the microswitches correctly. There are 2 and you orientate them with the button facing down. This should keep it working for another 8 years.