Tu te brosses les cheveux.

In this exemple, you’re just saying that someone is brushing his hair. This is called a phrase déclarative (but it’s informal here).

Tu te brosses les cheveux ?

Here, you’ re asking someone if they brush their hair. This is called a phrase interrogative.

The “?” is one way to show that you’re asking a question. They don’t sound the same orally.

You can say

• "tu peignes tes cheveux" or

• "tu te peignes" or

• "tu te peignes les cheveux"

  but you can’t say

• "tu te peignes tes cheveux" or

• "tu peignes les cheveux".

This last case is however correct if you add a complement.

• Tu peignes les cheveux de Marie, tu peignes les cheveux de tes enfants le matin, etc.

The same goes for "brosser". That’s just the way it is, you are here dealing with idiomatic phrases.

Both

Tu te brosses les cheveux.

and

Tu brosses tes cheveux.

are correct French but the first one is much more what people would spontaneously say. The second is still possible and might sound literary.

On the other hand, if cheveux comes with an adjective, the first form becomes unidiomatic, e.g.

Tu te brosses les cheveux frisés.

and the second form prevails:

Tu brosses tes cheveux frisés.