There have been some good discussion posts on the topic of 3-way brew solenoids lately.  I just wanted to add a little explanation of 3-way valves to help people understand how they function in espresso machines and problems you may see.

First, thanks to Terry Z and the Espresso Parts guys for putting up great pics of these parts on their site.  Most of the pics that follow came from there.

A typical 3-way solenoid valve is shown here --->

On many commercial espresso machines this valve is screwed directly to the group via the holes in the brass bass.

The valve serves two functions:

1. Start and stop the flow of brewing water

2. Exhaust excess pressure once the shot is complete.

The valve consists of four major parts:

1. The coil (the black box).

2. The valve guide "stem" (the stainless steel part shown at right).

3. The valve guide "base" (the brass part seen at right).

and...

4. The valve "nucleus" (seen here).

The nucleus is enclosed within the valve guide, and slides up and down inside of it.  Take a look at the nucleus and note the spring wrapped around it, the slots that run up and down, as well as the circular red valve seal. There is a second red seal on the opposite end of the nucleus, hidden from view in this picture.

Also take a look at the valve guide picture, noting the single large hole at the end of the valve guide "stem".  This is the exhaust port, which we'll call port #3.

The bottom side of the valve base is seen below. Note ports numbered 1 and 2.

Most commercial and some home espresso machines are plumbed-in (connected to a water line).  When they are, there is some some water pressure within the heat-exchange system or coffee boiler at all times.  Brew water circulates through pipes and channels within the group before entering the valve at port #2. 

When the machine is off or idle, the spring on the valve nucleus pushes the red seal against a valve seat inside of the base, closing off port #2 and keeping water from entering the valve.

When the barista pushes the button to start making espresso, electricity flows to the coil.  The coil generates a magnetic field, which draws the nucleus away from port #2.  This allows water to flow into the valve and back out through port #1.  From there, the water flows through the group and out the dispersion screen where it meets ground espresso.

One thing to note is that port #1 (the one leading to the dispersion screen) is always open.

In this way, the solenoid valve accomplishes its main function - starting and stopping the flow of brewing water.  If that were all it did we'd call this a two-way solenoid, but it does more that that... so we don't.

Recall that the valve's other function is to relieve the excess pressure from the brewing chamber when the shot is finished.  Port #3 is that exhaust path.

Remember that when the barista pushes the button to start a shot, the nucleus moves away from port #2.  When does, it closes off port #3.  This prevents the water that's now flowing through the valve from escaping through port #3.  When the shot is finished, the nucleus moves back against port #2, opening port #3.  This allows any pressurized water that remains within the portafilter basket to come back into the valve through port#1, flow through the slots in the side of the nucleus, then exit through port #3.

When the valve functions normally, only two of the three valve ports are open at any one time.

Though these are fairly high-quality components, failure is common.  Frequent use and constant contact with hot brew water and waste coffee create a pretty harsh operating environment.

How do these valves fail?  Coils fail.  Buildup or damage to the valve seals and seats often prevents them from sealing completely.  Sometimes, the nucleus will "freeze up", which will prevent it from opening.  The nucleus spring can sometimes break due to fatigue and age.  Debris or buildup can also obstruct one of the valve ports, preventing water flow.

Troubleshooting coils is pretty straightforward.  If the coil is "on" and receiving power equal to its listed rating (20V, 110V or 208-240V), you should feel a little pull when you (carefully!) tap the end of the valve guide with a screwdriver.  No pull? Bad coil.

Seal failure causes the valve to leak water, generally out of port #3.  Leakage when the machine is idle indicates a problem on the port #2 side, leakage while pulling a shot indicates a problem on the port #3 side.  Sometimes the problems can be fixed by disassembling and/or cleaning the valve, other times replacement is required.

There are two things you can do to prolong the life of your three-way solenoids:

1. Backflush regularly with a good espresso detergent.  Use only products that are labeled as "espresso machine cleaner", Cafiza, Puro Caf, etc.  Follow the procedure recommended by the manufacturer (though you should use less detergent, a dime-sized amount is adequate).  Please backflush often.

2. Make sure your water is "safe" for your machine.  Manufacturers generally recommend that water hardness not exceed 2-3 grains.  Use an appropriate filter and/or softener, and maintain them according to manufacturers recommendations.  Have your water tested to be sure that it is appropriate.

Hopefully this article has helped you understand the parts and function of three-way solenoid valves.  Please post any feedback or questions in the comments section.