You can also do like I did. Clean every joint and every drum cylinder in side around under etc. wrap what you can with a paper towel. Then get a book or iPad get in truck put the foot on the brake and hold pressure. Switch feet and keep pressure. Reading a book or iPad. (A nice block of wood wedged super hard between seat and pedal did same thing. ). After a good long while check every towel and location for a leak. I found one drum cylinder leaking. Since it was inside the drum I didn’t notice right away. I replace that one cylinder and it fixed my leak down and air. Also you can used process of elimination. Start caping a line or two and see if pedal gets hard. That will indicate where you may have air.
