Replaced a dryer and expansion tube and appropiate o-rings. Vacuum pulled to 28 hg mm, held for >20 minutes @ >25 mm hg. THus good vacuum and appears to be a slow leak. Added refrigerant 1.6 lbs (UV leak detector added also.) System worked well but 24 hours later entire system void of refrigerant?

Pulled vacuum again and holds for > 30 minutes @25 mm hg. No leak detected with BLACK light? What is the problem. I know I have a slow leak in evaporator (most likely) but why did refrigerant escape so fast!!

You will be able to see the dye so remove the glove box, knee bolster, blower resistor and look at the evap with a black light. If that's your leak, it'll light up like a christmass tree.
That's assuming you don't see the dye anywhere else under the hood.

I would think you would see the vacuum slowly rise, not hold steady for 30 minutes. There may be a problem with one of your o-ring installations, perhaps it seals during vacuum, but leaks when pressurized. Did you use a metal instrument like a screwdriver to get the old o-rings off? If so, you may have scratched the sealing surface enough to cause a leak. To see if the UV dye leaked from the evaporator, try getting some water to flow out the drain hose underneath the HVAC plenum, underneath the car. You might have to get to the blower motor, remove it, and spray some water at the evaporator and check for the UV dye when it exits the drain hose.

BECAUSE 850 EVAPS ARE PEICES OF CRAP, DONE MANY OF THEM.
GOT IT DOWN TO ABOUT 3-1/2 HOURS NOW LOT OF WORK BUT CAN BE DONE..