The spring brakes of a tractor-trailer will activate once air pressure drops to approximately:

20 to 45 psi.

First, the low-air pressure signal will be activated between 55 and 75 psi. Second, as air pressure continues to drop, the tractor protection valve will activate close to 45 psi. Third, the spring brakes will also begin applying at 45 psi as the spring pressure overcomes the remaining air pressure and will be fully applied when the air pressure gets down to 20 psi.

Source: Alaska CDL Manual