Chevy & GMC Vans

Just noticed yesterday on 2000 Chevy Astro 288000 km that coolant level in expansion reservoir was below ADD level.
Added antifreeze and water appropriately.
I now notice that my temp gauge is not sitting at the mid point of the gauge when up to 'normal' operating temperature.
Would a weak spring in the rad cap cause coolant to evaporate more quickly and cause the coolant temperature to be lower than 'normal'?

Several months ago, I added Barsleak or whatever it is called and this seemed to solve what had been a very slow leak eg. where the block heater enters the block.

Thks for any replies.

A rad cap is cheap insurance. But if you had to add leak stopper, chances are that there may be other problems. Even with a good rad cap, there will be some fluid loss.

thoughts:
first thought on the lower than usual temp gage readings is the thermostat is stuck open.
The Captain is right, minor loss of coolant over time is not unusual.
warm the engine up, shut it down and get underneath with a flashlight and have a look around. I've had a puncture in the lower radiator hose that did not leak a drop on the driveway but lost fluid under heat and pressure. Pull the reservoir and fill it up with water. let it sit for a while and look for leaks / cracks in the seams of the plastic.
the radiator cap could be toast, but if you are not seeing signs of leakage (dried streams of coolant below the cap) that ain't it.

Thks. Will do a few 'tests' as you suggest...