radiator

f you have an issue with the coolant system, the 1st thing to change is the header tank cap (MK1/2/3 Golf/Polo/Passat etc). The older black caps have a problem with the pressure valve in the cap which can vent to late/early, so its always a good idea to upgrade to a new blue one.

The system takes just over 5 litrs when empty, there will always be some coolant left in the block unless you remove the thermostat though
Use G12+ coolant from VW, or G12 from AVS Car Parts - [The VW - Audi - Seat - Skoda Parts Specialist]
Use at least 40% coolant, I normally use a 50:50 mix in my GTI 8v and it runs nice & cool

Bleeding the system after refilling:
Run the engine from cold with the cap off and rev it to 2k now and again. While you are doing this squeeze all the pipes you can get your hands on, and keep the coolant level topped up. In addition, whenever you rev the engine you should see a steady stream of coolant/bubbles spray out of the thin hose to the top of the header tank, if not the hose/rad is blocked, or the waterpump is bust. It will eventually start to boil over, at this point or when the hoses get too hot to touch turn the engine off. Once its settled down top it up to the max mark and you should be good to go!

Checking the operation of the system:
Check the lower rad pipe is stone cold until the temp gauge reaches the half way point, if it gets lukewarm straight away then the thermostat is stuck open or missing, if it stays cold then it is stuck shut.
Check the fan kicks in when the gauge gets near 3/4, if not disconnect the plug from the radiator sender in the bottom, passenger side of the rad & bridge the pins. If the fan spins then the temp sender is broken, if not jump the fan direct from the battery, if it still dont spin then the fan is knackered