Originally Posted by
JH870
Correct. At high engine speed the IP will provide unregulated air pressure which is then regulated by the bleed valve to 45 +/- 5 PSI. At low engine speed, the IP pressure is insufficient, and the HP stage provides 36 +/- 4 PSI. When the IP stage is capable of providing sufficient pressure, the HP valve closes pneumatically as a result of excessive high upstream pressure (ie. If there is sufficient IP pressure, there is going to be excessive HP pressure).
Thank you so much JH870. great explanation