Pressure in coolant system

[Manual Life]

This is for an 85 300D engine.

I viewed the thread posted by Dieseldan about this problem. Apparently I'm having the same issue as he was/is. I have pressure in the coolant lines after a short and long drive.

Here is the situation. The temp gage fluctuates some. Always between about 78 and 84 degrees but usually just below 80. Goes over 80 if I drive above 70mph but stays below under that speed. Given hose facts is that normal for these engines? Should I replace the thermostat?

Second, I've driven long trips and around town. Temp and pressure stay about the same. Coolant is 6 months new and flushed. No coolant is consumed. When I release the air from the over flow tank it smell like coolant and nothing else. And the pressure is gone over night.

Then, the bypass line use to weep but it no longer does. However, the overflow line weeps a tiny bit, as in very minor.

That's what's going on. I seek a solution. Any ideas? I would like to have this taken care of before the summer heat arrives.