[scottz]

He found the problem, or at least the solution. He had to start the process with both tanks connected (even though one was empty). After that, he could disconnect the empty tank and not have a leak. Must be something to do with they way the valve has to be sequenced. Here is what he did:

2 Scenariors:

Scenario #1
1. No tanks attached to system at all.
2. Attach full tank to pigtail #1, leave the pigtail #2 without a tank.
3. Aim regulator tank selector to tank #1
4. Open tank.
5. Propane leaks out pigtail #2.
6. Switch tank selector back and forth, no change in propane flow out of pigtail#2.

Scenario #2
1. No tanks attached to system at all.
2. Attach full tank to pigtail #1, attach empty tank to pigtail #2.
3. Leave tank #2 closed (since it is empty) and open tank number #1
4. Operate system (furnace in my case) to ensure LP throughout all lines.
5. Disconnect tank #2
6. No propane leaking out of pigtail #2