The solenoid bleeds vacuum when you turn off the key to lower the door. It will go down slowly unless you have the spring hooked up. That's the job of the spring, to assist in closing.
Some guys bypass the solenoid and leave the spring off so when the car is shut off the door will stay open.
Yes, you can by-pass it by either jumping the hoses with a nipple or run the hose from the canister to the switch if it will reach. You can use another nipple and extra hose if it doesn't. After doing this, if it still doesn't work pull off the other 2 hoses from the switch and using the Mighty Vac check for vacuum on both nipples while flipping the switch to the up and down positions. It should be the same as the vacuum coming from the canister. If not, there is a problem with the switch as I mentioned in my other response.
If it is low you can take the switch apart, I believe there is a gasket. You can buy individual parts or the whole switch to make the repair. Your welcome. Hope this helps.