One-Way Drain Valve

Automotive Mileposts
Image: Drain Valve

Above: One-Way Drain Valve lets water out, but prevents dirt, dust, and other contaminants out

One-way drain valves allow water to exit and prevent dust and dirt from entering

This unique sealing device is used on many Ford, Lincoln, and Mercury cars of the sixties and seventies. Normally mounted on the bottom of each door, they are sometimes found in the rocker panels and rear quarter panels as well. Made of thin, pliable rubber, these one-way drain valves have a plug on each end that secures it in place. When properly installed, the plugs position the rubber strap between them tight against the bottom surface of the body panel requiring drainage, covering the drain holes.

When water enters the door, it eventually finds its way to the bottom, where at least two of these valves are located. The water is able to run out of the door, past the rubber valve, as the strap part flexes to allow the water to seep past. This keeps the door cavity dry, and prevents corrosion from starting. The rubber strap covers the drain holes to prevent dirt and and other foreign matter from entering, thus keeping the interior of the door clean.

The rubber belt moldings at the base of the door and quarter window glass aren't intended to be water tight. They are designed to reduce wind noise and limit the amount of moisture that gets inside the door. These one-way drain valves are installed to allow the water that gets past the upper belt seals to run out the bottom.

These valves do eventually dry out since they're made of rubber, but reproductions are available. Automotive Mileposts recommends these valves are checked regularly to ensure proper operation, as they play an important role. Valves with dry or broken rubber should be replaced, and the drain holes checked at that time to make sure they're open and can drain water. Small rocks or pebbles can get caught in the holes, preventing them from draining properly.

If you hear water sloshing around in a door or quarter panel, see signs of rust or corrosion in the door or along the base of the door, or if the door or rear quarter cavity appears to be excessively dusty on your car, these one-way valves are the first things you should check. If yours are missing, dried up, or broken, they do play an important role and should be replaced. They are not expensive, and this is an easy job you can take care of quickly.