From The Porsche 914 Wiki

Hi, I'll be expanding this page over the next couple of days, bear with me...

Description

The 1970 and 1971 914's have a dash-mounted wiper switch that incorporates the washer switch by a central plunger. The washer system uses compressed air from the front spare tire to pressurize the reservoir tank. This system is notorious for air and fluid leaks, but works well when all of the components are in shape.

Photos

Testing

These switches are notorious for leaking. Before you install a new or supposedly "good" used switch, test it to make sure you won't end up with a lapful of washer solution.

A simple functional test is to take a hand vacuum pump and pull about a 15 inHg vacuum on each port, and watch to see if there's any leakdown. Depress the squirt button and pump again immediately to see if there's any leaking of the valve.

A much better test is to connect the valve ports to a regulated compressed air source. Set your compressor regulator for 45 psig (slightly more than the maximum system pressure of 42 psig). Connect to each port and test the other port with a wet finger to see if there's any leakage. ANY leakage will be an issue - though if you are using the chrome washer jets with check valves you may be able to tolerate some leakage. Also listen carefully to hear if any air is escaping the valve body.