2015 Tahoe LTZ - intermittent brake assist loss at low revs - vacuum pump??

[petethepug]

Low RPM means highest vacuum. If you have a vacuum hose coming off the brake booster, replace it. They deteriorate over time and can collapse on themselves under high vacuum.

Essentially slow speed, high vacuum, hose collapses on itself and then zero vacuum and hard pedal until hose unsticks.

 

[2015TahoePPV]

apparently when its running as a 4cy, the vacuum is low enough that the test engineers didn't like the brake feel when they simulated panic stops from a normal cruise speed, so the external pump was added. There's a lot of different conditions for the 4 cylinder mode to activate, but thankfully I've eliminated that problem.