Here is how to use the water brush pens with piston filler:

  The bristles are glued as they are new. First, you need to clean the glue by soaking and rinsing in cold or lukewarm water. Do not try to separate any part of the brush except the cap.

  The brush is now ready to use. You may use it as a regular brush; or go to the next step to fill the  reservoir with water. 

  To fill the brush, first, turn the piston knob at the end of brush handle counter-clockwise. Again, please do not try to separate the handle from the brush head or open the handle. Just turn the knob to  move the piston screw downward to drive out the air inside the reservoir. 

  Dip the brush head into the water and turn the piston knob clockwise.You should see the water is now sucked into the reservoir. Tip: if the reservoir is not full, you may point the brush tip to the sky and turn the knob counter clockwise slowly to push out the air, then, dip it into the water and draw the water again by turning the piston knob clockwise. 

  Now, you are ready to go.

  To change color, you may get some water out by turning the piston knob and absorb the color on a piece of paper towel.

The brush is very easy to clean, just squeeze out some water and brush it on a paper towel.  Main care tip for the water brushes would be to clean and drain them before storing them when they are not going to be used for a while.