As far as I know the yellow valve does not really have to pop until basically 0 psi and many trucks are set up this way. The reason being you want to have manual control of your tractor brakes as long as possible in an emergency situation and not have a "dynamiter" where the spring brakes all set up simultaneously. Which could definitely cause loss of control if you are still trying to get the thing under control while moving at a pretty good clip. This is not loss of a safety feature as the spring brakes will still come on gradually as the pressure drops below 40 psi or so anyway but you want them to stay off as long as possible to give you every chance you've got to fix whatever is wrong yourself.
Now the red valve on the other hand, is supposed to pop somewhere between 45 and 60 IIRC, too lazy to look it up right now, but always well before the yellow knob does to close the tractor protection valve and stop catastrophic air loss through the trailer.
And of course many yellow knobs will pop at pressures substantially higher than zero, there are 10psi, 20 psi, 40 psi, and even higher available, it all depends what you buy and install.
Birken