Sealco had a valve that was push to park,standard valve in Peterbilt trucks in the 60's, the knob was about the size of a silver dollar, thats all it did was park and release,below it was a push button with a knob about the size of a quarter it said push and hold for brake release, that was the emergency release knob.
What you may have John is the early Sealco plate that says push to park, and the valve has been changed to a newer style that is pull to park,a picture of nameplate and valve would be helpful.
