Have Boat Questions? Ask a Boat Repair Expert.
The line is one continuous line from the gauge to the pickup. The pickup should be located in the leading edge of the lower unit of the drive. A very small hole just above the bullet.
Tilt the drive up and locate the small black hose. There should be a connector a few inches above the mating surface of the lower unit, in the front. You can disconnect there.
It is very common for the opening in the lower unit to become blocked with debris.
Some installations also had a second connection next to the transom. If you hear a lot of air using the compressor then you may have blown the hose off the connector. All depends how your exact boat was built.
Run a thin wire into the lower unit opening see if that works.
Be careful to not use more than 10# ***** of air if blowing back towards the gauge. It takes very little air to damage it.
Check this out and post back what you find.