Excessive water comes out the drain when brewing a expresso

Hi
Your 3 way valve is dirty or bad. Have you ever done a back flush with a detergent?
About every 3 months it is a good idea to do a backflush your machine with a blind filter basket. When backflusing you should only use a good espresso machine cleaner designed for an espresso machine's brew system. DO NOT USE a decalcification cleaner. First, clean the screen and dispersion plate plus the brew head and reassemble, which will prepare you for backflusing. Then, insert the blind filter basket into the portafilter with a ½ teaspoon of cleaner in the basket. Next, place the portafilter into the brew head and tighten. When the machine is hot turn on the brew button for about 10 seconds and then turn off. Then, wait 30 seconds to a minute. Next, turn the brew button on again this time for only about 3 to 5 seconds and then turn off. Wait 30 seconds to a minute. Repeat these last two steps six to eight times. After you have completed the backflushing remove your portafilter and rinse the blind basket with fresh water. Next, you turn on the brew switch to rinse the plumbing. Then, reinsert the portafilter into brew head and turn on the brew button for about 10 seconds and then turn off. Next, turn on the brew button for approximately 3 to 5 seconds and then turn off. Repeat about 4 to 5 times. Always remember backflushing is hard on electric 3 way solenoid valves, so it is not recommended do this no more than four times a year for a machine used in home application. You will find more good is done to your machine be properly cleaning the brew head.