I've been working on a 1997 jetta GLS with some of my friends. This car sat for about 4 years in a garage because the master cylinder failed. We had to change the master cylinder and vacuum booster with the t1006a tool and got it back on the road for a couple hours.
It started having cooling issues when the black plastic housing for the temperature sensor cracked on our way home from the garage. The temp light came on and we shut the car off, and had to nurse it home waiting ten minutes starting the car driving a few miles and shutting it off and repeating.
We changed the black plastic housing and their are no more external leaks. At this point I wasn't aware of the pressurized cooling system. We started the car up and ran it around my yard and the temp started rising and the cooling fans wouldn't kick on without the AC button on..
I went inside did some research and figured it must be air in the system..so we started burping the system. Running the car with the cap off. There was no heat which was a good indicator of air in the system. I pulled the return hose for the expansion ball off of the head and cleaned that out. It was blowing only steam so I figured it must be venting air.
After running the car with the cap off around my yard the heat came back pretty well but its still spotty. The metal return tube on the back of the engine head that is the return for the expansion valve started spurting a decent amount coolant but still mostly steam... not any sort of consistent flow.. We figured that it would probably be ok to put the expansion ball cap back on. Shortly after it started to overheat again. We removed this cap and ran it for about two days down every bumpy road adding fluid, trying to get bubbles out and eventually its at the point where when we put the cap on the temperature gauge stays right at the middle mark no matter how hard you run the car so I hope this means its getting better.. The heat seems to be a lot stronger also.
A few things don't seem to be right:
- The low coolant light is coming on even when the expansion ball is full. I've read that's pretty common and usually requires replacing the expansion ball. Alright no big deal right?
- The fan's still do not work without the AC on.
-The upper radiator hose though it gets hot seems pretty soft and lifeless. If you completely close it off with your hand in a glove you can feel it slowly expand like a balloon but not dramatically and it never gets firm... 
-We flushed the radiator it flows like a champion so it's definitely not clogged. If you remove the upper radiator hose from the radiator and shoot a garden hose down the upper radiator hose towards the head the water seems to go through the head/block/lower hose back out the upper radiator hose fitting.
-Next piece of the mystery the metal tube coming off the back of the head that is supposed to be the return hose into the expansion valve is now only putting out steam again... I thought maybe this is a bad headgasket but the oil is fine, the plugs look fine, and there's no coolant in the exhaust..
-The heater core isn't leaking, floors are not wet. Might bypass it incase its obstructed just for troubleshooting purposes..
Do you think if I blow compressed air in the metal tube at the back of the head for the expansion return it would free up anything obstructing the system?
could a water pump impeller slipping on the shaft or a partially frozen thermostat cause any of this? There are no physical leaks and no signs of any internal leaks.. The way that plastic flange for the O ring on the housing broke makes me worry there may be a large chunk floating around in the cooling system possibly even obstructing the head.. I'm thinking about removing the upper radiator hose at the black plastic temperature sensor housing and blowing compressed air through the small hose return line on the back of the head then flushing the head with water.
I'm at a loss. I've definitely added more than 6L of coolant, the O ring on the pressure ball is good and i'm not losing any fluid from through that. Maybe it was just massive amounts of air or air getting stuck in stupid places i'm not sure but I really need some direction/suggestion/input... anything would be appreciated.
The temperature gauge seems to be holding steady at the middle mark with the pressure ball cap on .. This has definitely driven me pretty darn crazy.. regardless I would expect more coolant coming through the return line coming off the head if it was working properly. Nothing but steam...
1997 Jetta GLS 2.0 Extravaganza! The mysteries of the steam Pt 1
Picking up a new one from autozone for 30$ wish me luck! 
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