Dear Steve and Larry,
Got the receiver/dryer (brand: "Hansa", price: USD$14) replaced today, together with the 2 pressure switches (high pressure switch and low pressure switch). Vacumm pump was indeed used in the procedure before charging with R12. My mech did not have a machine to store the R12, so he leaked it into the atmosphere, and so had to charge new gas into the system. (a waste of money though)
One interesting point is that it cost me only USD$11 to charge up fully with R12. My understanding is that it cost much more in the US, which is one reason why many people are converting to R134. R12 is readily available here, and is rather cheap. I was charged slightly more today for the R12, usually I pay about USD$8.
Thanks to you guys for the invaluable help. I could not have learnt more without your advice. Cheers!
1983 W126 (280SEL)