Likely the Toro website has a Q&A for that question.

They don't. I called them, tho, and got the answer. Thanks.

Pleased to hear that!

so... what was the answer?

It depends upon the position from which you blow. In my case, I am blowing upward (connection point for compressor is BELOW the PVB. The PVB thus stays OPEN. Which is what I did every year. THE PROBLEM turns out to be -- drum roll, please -- the compressor hose. It's too long. It took me a bit to figure it all out. But I went on a sprinkler forum and read a post from a guy with the same problem: lines wouldn't blow no matter what he did. He mentioned using some long 3/8 inch hose and was told that that could be the problem. I didn't put 2 and 2 together until the next day when I stood there scratching my head, befuddled. Then it hit me. My hose is very long. The hose I used to use every year prior was very short. The air loses pressure traveling thru and never builds up enuf to pop the heads. Who knew? Live & learn. :)