When I adjust the windage on the rear sight, instead of staying put, the aperture flips to the opposite peep hole, depending on if I'm moving the windage left or right. Once changed, I can adjust the windage correctly, but I then have to flip to the aperture I really wanted. I don't know if the screw is binding it what, but it's hard as hell to move the windage wheel and I cannot change the windage without the aperture moving along with it. Is there any way to fix this problem of should I just get a new rear sight? Are they a pain to replace? Thanks.

It sounds like the threads are too tight between the flip apeture and the windage screw. If you want to take it apart you have to knock out the roll pin that holds the windage knob to the windage screw. You have to have some support on the back of the windage screw as you knock the roll pin out or else you may bend the screw.  You need to be careful of not losing the small detent ball and spring underneath the windage knob as they will fall out when you remove the knob.  I would try to smooth the threads out with an abrasive paste and oil or just find another apeture or screw or both.  You can get a finer threaded one (Match type) for finer adjustments now that you have it off.  

Putting it all back together can be a little tricky as you have to line the holes for the roll pin back up and fight the detent spring pressure and again have a backing so you don't bend the screw.  One of those four handed jobs.  It can be done though.  It also helps to have a roll pin starter to install the roll pin.  I got a set from Brownells.
 
I believe the Derrick Martin AR-15 book has a good description of the whole proceedure.  Look at a parts breakdown as that will give you a good start on how all that is put together.  

Have you considered just sending the upper back to the manufacturer for warranty repair of the sight?  I know it's a pain to go through the expense and wait, but that is an option.  

I've swapped parts out on my sight and it is not a big deal if you are careful and have the tools to do it.

You do not need to remove or replace the entire rear sight assembly.

A 1/16 roll pin punch can be used to
carefully punch out the roll
pin in the windage knob.
Then you can remove
the the aperture and knob
which will allow you to check the threads for damage or need of correction.
Also don't lose the ball bearing that indexes the knob.

This is the never ending ASA upper story. They finally gave me this one after the first one was "repaired" after the ejector disintegrated. But the repaired upper still jammed constantly, so I sent it back and they gave me a new, complete upper. So, this is the 3rd try. It's unlikey I'll send it back again. In fact, this is the best one so far. No feed problems like the first, very accurate with tight groups, etc., seems like a solid, reliable upper. I'd like to avoid sending it back as they might replace it with something worse.

Brownells has roll pin punches. Should I just go ahead and buy one? Are there any other tools I need to do the job correctly? Does the PS/2 tech manual explain the procedure? I think I'll just replace the entire unit.

If you replace the whole unit
just remember that the pins and punches
are steel and the receiver is not.

Removal of the entire unit is easy.
Putting on new one on is not too
hard either.
Just keep the elevation spring compressed
and guide the pin through.
Do it carefully or you might enlarge
the pin hole.
...............................
My basement where all my
tools and fixtures are
is way too dark for good photos.
anyhow this is all you will be up against:

Remember to place the spring and
ball in the small hole
before you start. They will be held in place against the receiver after
the elevation knob is turned enough
to lower the sight assembly.
[img]http://home.earthlink.net/~thegardenweasel/sight1.jpg[/img]

Keep the elevation spring compressed
and guide the roll pin through.
[img]http://home.earthlink.net/~thegardenweasel/sight2.jpg[/img]

If your elevation knob
doesn't bottom out at either 8/3
or 8/3 - 2
you can remove the small allen screw
on the knob; adjust; and replace.

Amen, brother!  The right tool is essential, be it a firearm, car or mowing the lawn.  I hate repairing a car someone worked on with cheap or incorrect tools.

Good luck,

Eddie

garden weasel, thanks a lot for the pictures and the advice. That's a major help!

Is a roll pin starter the same as a roll pin punch? I think I saw both mentioned at Brownells.