Just wanted to let you know I have the same exact symptoms and problem on my wife's 1991 190E 2.6L.
I don't have any guages to read the pressures on the system but the compressor runs constantly, no cold air, high side lines are hot and low side lines feel like ambient temperature. Also bubbles in the sight glass.
I haven't been able to figure it out either. I know this reply doesn't help you at all. But i feel for you...
Keep me posted and I'll do the same,