I've now replaced the orifice valve on my 78's A/C three times, with the last one taking an HOUR to get out!  The A/C accepts r134 with no issue, but the tubes only get cold up to the tube that accepts the orifice valve.  The tube above the valve never gets cool.

I'm thinking that I may be putting it in upside down? 

Normaly, the orifice tube will only fit in the line one way. The high pressure lines will always be hot: the suction(low) side will be cold after the orifice. This is the point where the pressure drops, the freon expands, and gets cold. If the system is working right, both of the lines on the evaporator will be cold, but the one with the orifice will be much colder. The line up to the orifice should be hot, as the freon is under presure at that point, and pressure = heat. As far as whether the tube is in backwards or not, it really won't make a difference, as all it is is a very small tube, with screens on either end. They don't have any moving parts, or valves, just a hole. If you have managed to put one in backwards, please turn in your "Certified Mechanic" card, and step away from the vehicle!
If you look at the orifice tube, you will see an arrow on it. That tells you which way to install it in the line. As a rule, the longer end of the orifice goes into the line first.The end that has the o-ring on it will be closest to the threads on the line. There is also a small "crimp" in the line for the orifice to seat against, and the is what keeps it from disappearing into the evaporator core. This also makes it very hard to put the orifice in backwards.
I assume the A/C is not working, or you wouldn't be going thru all this? (or do you just need the practice?

Well, I'm getting plenty of practice, but I really would rather be practicing other things instead!

Right, the A/C's not working still.  I'm really not sure which way the arrow on the valve is facing, but from what you've described so far, I've put it in correctly.  The one I just took out yesterday somehow had gotten pushed down about 3" into the lower end of the line, had to really think "outside the box" to figure how to get it out (ended up using a 6" deck screw, worked like a charm w/out damaging the line).

I'm without a doubt an idiot when it comes to A/C systems.  They took my A/C Repair Tech card by force a long time ago..lolol. 

I know my blower is on the way out, makes a slight thumping noise when on low, but is still working.  That wouldn't cause the air not to blow cool when on MAX, right?

Are you evacuating the system before charging it? It may be a dumb question, but it needs to be asked!
The blower making a noise won't have any effect on whether or not the system is cooling.
Does it cool at all, or not at all? What are the high/low side pressures? So many questions, so little time!
Have the orifices you've taken out have any crap on them? If so, then you may have a contaminated system that needs to be flushed and evacuated, after finding what contaminated it to start with, and correcting that.

When I first got the car, I had a pro look at the A/C system.  The original orifice valve was contaminated.  I then replaced the compressor, drier (think that's what it's called, small tank like thing), and orifice.  Before putting the new stuff on, I flushed the system with A/C flush, then blew air throughout the entire system to make sure all the flush was out.  Then put all the new stuff on (after adding A/C oil), and tried charging the system. 

No idea what the pressures are, using the allegedly idiot proof stuff from Advance Auto Parts.  No cool air at all.  Since I flushed the system, all old orifices have been clean.

But....is the system being evacuated before charging?
You have to evacuate it to get all of the air & moisture out of it. Otherwise, it will never cool. You say you flushed & blew it out with air, but no mention of evacuation.
If you have not evacuated the system every time it's been opened, that is the reason it won't cool. You'll need to take it and have the system evacuated, and then when it's recharged w/freon(134), it should work.
Evacuation is the process of connecting a specialized vaccuum pump to the lines, and creating a heavy (30inches) vaccuum inside. Depending on the pump, it will take anywhere from 30min. to an hour to do a good job. This gets all of the air out, and also, the vaccuum will release any moisture in the system, and draw it out. Then you have room for the freon & oil charge.

aaahhhh.... so I guess that this is a total fallacy beleiving that A/C recharging is a back yard job.  That must be the reason then, since everything else seems to be working ok.  Good thing those orifice valvles were only a few dollars, since I kept thinking they were the problem!

So I guess there's no way around my spending the $150 or so to have someone else do the recharge then.  Thanks for the info!

Nope you gonna spend a few $$$ on this one but it's worth it...Especially on those REALLY hot days....

I'm glad I did mine...

[QUOTE=Lockdown]

aaahhhh.... so I guess that this is a total fallacy beleiving that A/C recharging is a back yard job.  That must be the reason then, since everything else seems to be working ok.  Good thing those orifice valvles were only a few dollars, since I kept thinking they were the problem!

So I guess there's no way around my spending the $150 or so to have someone else do the recharge then.  Thanks for the info!

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Well, no......you could buy a vaccuum pump, and charge your buddies to do theirs!
A good pump runs around $200-300, but it will pay for itself if you do enough A/C work. I don't normally do any A/C work at the shop, but I do all of the work on my own vehicles(6), so it's been well worth it over the years.
You can take you car to a place that will evac. for you, but some places don't want to do it, because the real $$ is in the complete "service", ie: drain, flush, evac., charge. I'm sure you'll be able to find an honest place to do what you need. I bet you'll be "chillin" then!

Anouther neat use for a vac. pump is when you're trying to find a vac. leak in your headlights, or wiper system. You can run a vac. hose to the pump, and let it be a continuous source of vac. that won't leak out before you can find a leak! Adams' Apple38558.7764930556