Brew head leaking - brew button problems - Wand working?

When my Starbucks Barista is powered on ( Brew and Steam buttons are NOT pressed & Steam Knob closed tightly), water immediately starts flowing from Brew Head and doesn't stop UNLESS machine is powered off OR the steam knob is opened which makes water then flows out of the wand. I press the Steam Button it will heat up and green ready light will go on and I can steam milk BUT as soon as I am done with frothing , close the Steam Knob and press the Brew Button- it immediately starts dripping from the Brew Head again then it will pull a shot, but won't stop pulling the shot -(even though I depressed the Brew Button again) -only by powering off the machine will it stop brewing. Suggestions for self-repairing this problem would be greatly appreciated. P.S. I have descaled the machine using a Swiss descaler product, I have checked the springs in the portafilter (ok), looked at the gasket behind the shower screen (looks ok) - still having the same problem.