Do you have a 1.4 bar (140 marking) cap on the reservoir? How would you see it overflow? A hose goes from the neck to a bottle in the fender.

The aux fan trigger switch on the 124.133 is rated for 105*C. It's the 3-pin switch atop the turret to which the upper radiator hose attaches. I forget which pin pair to jump to test the aux fan. It won't hurt to jump pin pairs to test the fan. One pair triggers the fan (105*C), another pair cuts the compressor (IIRC 127*C in a 124.133, say what???), the third pair does nothing. An 86-87 switch has a lower AC cutout (120*C?), an early 300E switch has a lower still AC cutout (115*C) and a 190D (red top) switch is the lowest I've found rated at something like 95/110*C. Yes, the 190D switch has a lower aux fan trigger which helps cabin cooling on hot days. And when the temp gauge reads 110*C, I won't need AC since I'm not going to drive the car further.

Sixto
87 300D^2