There is a rubber covered button the end of my combination valve. My valve has been tripped turning on my brake light. I have tried pushing the button thinking that it would reset but nothing happens. Can someone tell me the purpose of the button or how it functions??

The button is for assistance in pressure bleeding the front brakes. It is a manual override for the front brake delay (metering) valve used on disc/drum systems. Pressure bleeders operate within the range where the valve would cut flow to the front brakes, therefore the presence of an override button.

The only info I could find for ya is in my 71 chassis service manual,69 should function similar.Quoting from the manual;
WARNING SWITCH
The warning switch is the pressure differential type.it is wired electrically to the warning lamp on the Inst.panel to warn the vehicle operater of a pressure differential between the front or rear hydraulic system.Once the switch is activated "on,"it will not "reset" until the defect in the hydraulic system has been repaired.Hydraulic reset to the "off" position occurs with the application of equal front and rear pressures.

Did you ever get around to replacing that leaking MC ?Hope this info helps.

I replaced the master and everything at this point seems to be better. I have yet to bleed the brakes but just testing the pressure in the master compared to the old was a lot better and the light from the comb. valve did go out. I'm thinking that after bleeding the system everything should be a bunch better.