If it's truly a muddy brown.. usually in a GM it's caused by the factory-installed stop-leak tablets. If it's another manufacturer or if it's a redder brown like rust, there are a few other possibilities.. oil leaking into coolant, coolant so old or weak that the system is rusting, or something was added to the system without your knowledge (stop-leak or other).
there is supposed to be water in the radiator. your radiator is supposed to be evenly mix 50/50 with radiator fluid and water, preferably distilled water.
There is a water reservoir tank that goes to the radiator that you can add the water to.
Radiator coolant.
You can add water to a radiator any time it is low on water.
remove your radiator form your car then poor some hot water in radiator till water comes out nice and smooth.
The radiator cools the water which then goes to your engine to cool it off, as cool water enters your engine hot water from the engine enters your radiator
The water is boiling in radiator because it's temperature(radiator's) reaches the boiling point of water when the engine works for a long time.
When a car is hot, keep the car running and add water to the radiator. When a car is cold, you can simply put the water into the radiator.
radiator fluid
To add water to the radiator on a new beetle, first remove the lower radiator hose. Then add water to the coolant reservoir.
Remove the bottom radiator hose. Stick a hose in the top of the radiator. Turn the water on. Allow the water to run through the radiator and out the bottom of the radiator.
Number one, replace the water pumb. Usually when water leaks from the pump it is through what is called a seep hole and signifies the pump is bad. That brown fluid is probably water with alot of rust mixed in it. Good idea to have the radiator flushed and filled with new coolant. rusty water, change the water pump