first loosen the line between the filter lockoff and vaporizer, then pull the vacum hose off the mixer body that leads to the fuel lockoff/filter and put vacum on it. If you get a rush of lp gas out of that line then you know your getting lpgas and the lockoff is working properly. If you get nothing then your rebuild went wrong and you should go back in it and re-check it.

Since you said the bubble trick failed when you put the supply line from the vaporizer into water and got no bubbles it seems it would be safe to say the vaporizer might be your problem but i would not rule out the lockoff until after the test ;).

Personally i haven't had very good luck rebuilding vaporizers, first because its too time consuming and labor costs just dont justify that that compared to just swapping one out.
If it were me? i would get a new (cobra style) vaproizer without the primer button and just swap it out. Why do i say to use the new style vaporizer? because they have made significant changes in its design, (one change being a new style reed valve), from the model j and it is a better unit i think. The operation of the truck will not change, you just wont have the primer button to play with :)

Good luck