Two Years?!!! when there is a coolant leak the level goes down. When you drive the vehicle the air pocket overheats causing steam which expands and pushes the little bit of coolant remaining back into the reservoir. You don't get an overheating temperature gauge because there is no coolant touching the temp. sensor..the air pocket is there. Bottom line is you are overheating your motor and by now probably have a bad headgasket. If you've only been adding water you also have a water pump seal that's been worn and may be another spot that is leaking. You'll find out soon enough! Keep your cell phone charged...you're going to need it to call for a ride!

Look at your radiator cap. See the PSI rating? At that rating, the coolant (and any trapped air) is vented into the overflow tank. That car is too old to have a hot/cold line I think (don't recall seeing that on my 80's GM vehicles) but as mentioned, coolant expands as it heats. Expanding creates pressure. When it "vents" the overflow, the level raises. When the coolant in the cooling system contracts (as the engine cools off) the radiator cap (it's now acting as a one way valve) allows coolant to be sucked in from the bottom of the overflow (thus not air, just coolant) until the temperature equalizes.

Very simple, not understood by a whole lot of people.

Will Therussaian the non-mechanic is all wet! Fill the radiator to the top when the engine is stone cold. Fill the plastic coolant recovery reservoir 3/4 full at the same time. If you did what Will said you'd puke coolant out the recovery bottle as the engine gets up to operating temperature.

My buddy has an 87 El Camino with a 450 hp. 383 and this is exactly how he originally filled the radiator and coolant recovery reservoir.

Depending on a car! some cars have pumps, that circulate the coolant when the car is on! so it seems like there is less coolant when the car is off and it all settled! Try filling your reservoir to the top, while the car is idling...Notice! Do Not to fill the radiator, when the car is on as, the pressure and high temperature can cause hot coolant to spur out! Also run the car for 5-10 minutes and check for any leaks, underneath! And check your dash for temperature gauge, if your car is overheating! Most cars have it!

When you drive and the engine heats up the coolant expands and goes into the overflow reservoir. When it sits at night and cools down the coolant contracts and doesn't take up as much space. LOOK at the reservoir and you'll see two lines. One marked "Full Cold" and one marked "Hot fill". When the engine is cold, in the morning, add coolant until it is up to the "Cold Fill" line.

inspect the plastic tank for the overflow, it should have warm or hot, and cold levels.

Using a light can make it so you can see that.

Using a sharpie you can set where it is at that temp for simple reference when you forget again.

If it does something out of spec, you can see it easy without getting messy or handling the coolant.

Fill the reservoir to the 'full cold' line which is usually around the halfway mark.

Depending on the car some have the ability the get rid of it

Heat makes things expand

Sound like it's low on water . when it get hot it expends, now you see it now you don't . Service as needed !