THIS IS A Reply to Author THAT I COPIED TO POST HERE IN THE THREAD.
Tim mentions slip fittings. If those are what I think they are -- I
tried those the last time I replaced a valve, and they slipped all
right! -- right off the pipe -- luckily I was watching when it
happened: GEYSER!! But, maybe they were some other kind of pressure
fitting.

On Apr 24, 3:48 pm, Neil wrote:
First I'll remove the old valve by cutting the pipes, then glue on new
3/4" PVC pipes to get the new valve well above ground level. Then
screw 3/4" fittings into the female openings of the valve. NOW, HERE'S
THE QUESTION: it seems difficult to get the fittings onto the pipes
because twisting is impossible. Do you just put on a lot of blue glue
and quickly shove both sides (valve and antisiphon) on? Twisting is
supposed to give a better seal. One way I thought of is apply the glue
in all 8 places (both sides of each pipe and all 4 fittings (2 for the
extension and the 2 in the valve unit), have a helper push down on the
valve unit while I hold the extension pipes, one in each hand, and
twist them both into their top and bottom fittings at the same time.
And we get only one chance to get it right. Advice appreciated.

I just looked at one I had done several years ago. Looks like my two
pipes screwed into the anti-siphon valve. I attached a male/slip
fitting to each pipe and put them into the anti-siphon valve,
tightening most but not all of the way. Then I used a slip fitting on
the receiving end. Put solvent/cleaner on the loose ends and attached
the whole apparatus. With the slip ends of the piping inserted I
could tighten the threaded ends and rotate the slip ends
simultaneously.

Alternatively you can just use a lot of solvent in the hopes that the
excess helps to bridge any gaps that screwing would have mended. Or
you could use Union pieces, although they're a little more expensive.
The union would allow you to disassemble if you need to replace it.

Another technique is to assemble the whole unit in advance with elbows
at the bottom of the pipes emanating from the valves and then bend
them slightly to fit in to the main pipes.

Just ideas off top of my head.

Tim