Yesterday I checked coolant level in reservoir tank. I had to fill 0,5 litre (1/8 gallon) water to rise coolant level away from min. level.

At the same time I study the cap on this tank. It had a ventilation hole (small finger size) at top. I searched forum and found that this is correct.

But it didnt had any valve or any pressure regulator her. Just a top-lid.

Is this normal/OK for cap?

And: With this solution you got to expect some evaporating of coolant from reservoir tank, aint ?? (Even if it has no pressure or very high temp.)

 

Just a cap is normal as the pressure is controlled by the radiator cap on the radiator. My cap doesnt have a hole in it. Its just a normal sealed cap.

 

My 89 and 86 both have the hole. It would just blow the lid off if you ever overheat and you'd have to find it. You're not going to lose much fluid over such a small hole.

 

Well think about it for a minute. If the holes were not there, the air in your overflow tank would not vent and make room for the coolant when the overflow tank fills up.