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Question:

Before you drive a vehicle with a dual air brake system, wait until the pressure in each system reaches at least

A 100 psi.
Explaination

Before you drive a vehicle with a dual air brake system, wait until the air compressor has raised the air pressure to at least 100 psi in both the primary and secondary systems. Watch the primary and secondary supply pressure gauges (or needles, if the system has two needles in one gauge).