Hi ! I'm going to "bench bleed" my new master cylinder and I'm reading the instructions.....It says : On the disc/drum type brake system make certain that the METERING VALVE BUTTON is in the "off" position until all air is removed from the system. WHAT IS THAT AND WHERE IS THE BUTTON ?????

I have a 1970 Chevelle with disc/drum system.......

On the original equipment 1970 brakes there is a round valve next to the master cylinder connected to the front brake line.
On the back of this valve is a button which must be held when power or
vacuum bleeding. It should not be necessary to hold it if gravity bleeding or using two people. It will block fluid flow when between certain pressures.

Some cars have had this valve removed, or never installed with a conversion. Some kits will not contain this valve, or will come with a combination valve. The combination valve can also have the button.

OE metering valve:
http://pg.photos.yahoo.com/ph/pearlfootprints/detail?.dir=d9df&.dnm=4b05.jpg&.src=ph
http://us.f2.yahoofs.com/users/3fde09fd_13533/d9df/__sr_/4b05.jpg?ph.c5fCBnFJ7PoLS
button is shown under black rubber boot to left side of valve in pic.
When installed, this button is towards the firewall. The rubber may no longer be there.

Aftermarket metering valves:
http://www.ecihotrodbrakes.com/images/ec-526.jpg
http://www.mpbrakes.com/vl3526.jpg
both showing button under black dust boot on top of valve in pic

GM style disc/drum combination valve:
http://www.mbmbrakeboosters.com/Images/web-catalog/images/PV2_jpg.jpg
button under black dust boot on right side of pic.

GM tool J22793 made to hold button while pressure bleeding on 67-70 style valve. (http://www.1968firebird.org/68sm/discbrk/discbrk6.jpg)