Many "receiver driers", used in the high side (the smaller hose) are compatible with both R134A and the old R12. So the one you get is likely to have a sight glass, BUT!!!
You can't charge by adding R134A till the sight glass clears, like we used to with R12. The flow and the look don't let you see what it's doing.
Charge by pressures. LOW (Suction, Large Hose) should pull down to about 20 and HIGH (Small Hose) shouldn't go much higher than 2.2 x ambient pressure. So if it's 95 out, HIGH shouldn't be much higher than the low 200's.
Here's a pic of a Receiver Dryer cut open. Notice it's directional. Should be marked with Arrows or In and Out. The OUT has a tube that extends to the bottom of the "can" so only liquid refrigerant leaves exits the Dryer on its way to the Expansion Valve.
OH! Be sure the new Thermostatic Expansion Valve (TXV) is wrapped with insulation, usually called PresTite. It'll keep humidity from condensing onto the TXV while it's in normal A/C operation. Dripping condensation is the least of the problem. The condensate will freeze and the ice causes the valve to stop the cooling process!
