Have you checked the Gasket that goes around the Trunk opening?
It has been damaged or the seal has deteriorated
I had the same problem with my 1992 Accord. Replacement of the trunk lid gasket fixed the leak, but I had to special order the gasket from a dealer.
A bad trunk seal weatherstrip.
Bad rubber seal around trunk lid?
I pulled out all the little rubber plugs on the trunk lid. Problemed solved.
There are many possible leak points, but the most common place for a trunk water leak in a neon is around the tail lamp mounting bolt holes. If you are unable to locate and repair the leak, there are people who specialize in this type of repair.
It means that you have a leak somewhere and the system can't keep to maintain vacuum. If the level of fluid does not drop it does not mean that you do not have a leak. It did not have time to develop.
If it were mine, I would have someone get inside of the trunk(boot)with a flashlight. Close it and then run some water over and around the edge of the trunk; not too much pressure. Start at the bottom(closest to you) and slowly work your way around one side to the other and then back to the bottom. In other words, make a loop around the edge of the trunk. Your helper should be able to see where the water is coming in, if there is no leak then it might be around the taillights or rear window. If the leak is around the trunk weatherstrip, either replace it or close the gap at the leak by hammering the lip of the trunk opening, not the trunk lid itself. If it around the taillights or window, try using some silicone sealer.
Depends on where the water is? back seat? trunk? floor board? mine has a leak from the driver door seal being weak, the trunk seal the same and the rear water channels for the sun roof need to be blown out again to stop the back seat leak when parked on a hill and it rains. so to be honest you have to find out where the water is, your best guess on how it gets in (ie light rain, heavy rain, car wash etc) before people can truly help you.
Dealer or body shop. Have you tried getting in the trunk with a flashlight and having someone pour water on the rear of the car.
If it runs good then one of your hoses is clogged up.
If I did it would be because there is a flaw in the gasket that seals the trunk lid. To find it take a flashlight and have an assistant with keys close you in the trunk. Have your assistant run water from a hose over the trunk lid while you look at the seal from inside. You will find your leak.