The metering block is designed for cars with disc/drum set ups.. It slows the front disc brake action down so the mechanicals of the rear shoes can catch up and the brakes apply evenly..
I never heard of having a metering valve on a all drum set up.. That's not to say it can't be done though.. I would look into that..
Also, is the master cyl plunger all the way seated when the pedal is not in use..
I get a lot of good suspension and brake info here... http://www.pro-touring.com/forum.php