Pure foam after 20 glasses so.

[Bobby Birdsell]

Help!, I built a portable tap bar with four taps from instructions found online. It works great for about 20 glasses of beer and then all I get is foam. The mini kegs are setting in ice, the pressure is at 12 lbs. I have been told it might be a bad regulator. I have used it three times the first time it worked great. The 2nd and 3rd time is when it started the foam issues.
anybody know what's wrong?