brake pedal still seems to low feels like only the fornt discs are working

[shadowgray396]

In 1970, the distribution block is the same for drums or discs.That is what is down on the frame.
http://inlinetube.com/Prop%20Valves/PR-107.htm

In 70 on the Chevelle had a proportion valve (or some call it a metering valve) just below the master cylinder and a pressure hold off valve in the rear by the rear axle. Not sure what the ElCamino had.

You can replace the distribution block with a combination valve from 1971 & up model, which incorporate the metering and proportioning valve into the distribution block. Note that the fitting sizes aren't the same between the two.