Air Brakes on trains do not require any air pressure to be released.
http://www.railway-technical.com/brake2.shtml
Ergo, while the brakes are applied, the
Auxiliary or Brake Reservoir can't be charged while the brake pipe pressure is reduced - as is the case when the brakes are applied.
The link is an excellent document describing the air brake systems on trains, I have only quoted the opening paragraph.
