Their is a couple of automatic ones out their.
I have the Staun deflators set of 4 think they were $40, you get to the beach screw them on I have them set to stop at 15psi put on waders as the air come out take em off and set to go.
I did make my own with a large paper binder, a hole drilled in the top with a small enuff bolt that would fit in the valve to let air out, dont rember the size mine was from a old erector set a nut to hold it in place. Start counting to when putting on first one after a 30 count take em off and check and adjust.
Now if their was just something that did it when airing up.