It has been a couple of years since I topped off my R12 in my 84 190E.

Before I put my manifold/gauges on, which was the HIGH pressure nipple, and which was the LOW side. If I recall the low was up near the left (drivers) strut tower and the high side nipple was over near the oil dipstick / exhaust manifold (right side)

Thanks

(I know to fill on the low side!)

An easy way to be sure is to run car & see whitch one gets hot! the hot one is the high side. Also the high side uses a *smaller* dia tubing than the low side.

Hi Lo
 

Brother of the Benz
In determining what line is what, remember that the low or suction line is the larger of the 2 and returns from the evaporator/dash to the compressor. When the A/C is operating this line will be cold. The discharge is the smaller of the 2 and goes from the compressor to the condenser. This 1 is HOT.
Your charging hoses must have valve openers.
If you are capabile of servicing your unit, and considerring the cost of R12; you may consider changing to R134A.
The change over is quite simple and very inexpensive, $29.95 at K-Mart. This includes 3 cans of R134A, 1 can of oil charge, a can adapter and charging hose with excellent instructions.
My W124030 does quite well with R134A.
Happy Trails Beep Beep from The Spiderman in Houston!!!