What purpose does the valve that mounts beside the master cyl. serve? On the rear of the valve there is a rubber boot that covers a valve needle of some type, is this some kind of a bleeder or reset? Is there ever a time when you would push it in? This is the original one that came on the car. Thanks John

That is a hold-off valve. It delays the front brakes just a little so that that the rear brakes are activated just a little before the fronts come in. It keeps the rear end of the car from coming around in hard braking. The little push valve bypasses this so it is easier to bleed the front brakes.