89 300E coolant issue

[think300E]

My car uses oil like most other older cars - its an ongoing issue.

During a typical stop to put some oil I noticed my coolant level was right above the sensor. (too low)

Also noticed that the windshield water reservoir was leaking from one of the bottom connectors (right hand side) which has a hose that goes to left hand windhield wiper (headlight).

Windshield water reservoir liquid was hot.

Put some mbz coolant/water to proper level, and drove to my destination.

When i got there noticed the level for the coolant was once again low, and the car was still leaking from the water reservoir.

6hrs later when i checked the car (after being parked for that long) the coolant level was back to normal.

What I believe is happening is that for some strange reason, the coolant is making its way to the winshield water reservoir as it expands, and since the water reservoir is old (and possible not meant to handle hot liquid) it started to leak.

When the car has cooled down the coolant ens up back at the reservoir.

To resolve the leak I will need to replace the reservoir, but what about the issue of the coolant making its way to the windshield water reservoir?

Please advise.