The optional master cylinder kit provides all you'll need to change to a disc/drum system. Notice, the proportioning valve goes in the line to the rear drums. Replumb the brake lines by plugging the rear of the original "T". The new line to the rear goes from the rear port of the m/c through the proportioning valve and reconnects into the old rear brake line.
The optional dual-chamber master cylinder is a direct replacement, on the two existing studs, on the stock '60-87 GM and GMC pickup firewalls.
As you noticed in the brake kit photo (above left), a 60-inch length of 1/4-inch brake line, a 1/4-inch union and a 1/4-inch plug are included. The original 1/4-inch supply line from the original master cylinder should be installed in the front port of the new dual chamber master cylinder. In the rear master cylinder port, install the short 1/4-inch line with 2-90-degree bends in it and attach the proportioning valve to the end of this short line to the port marked "IN".
Next, install the 60-inch brake line in the "OUT" port of the proportioning valve and route it to the right side of the frame just behind the rear of the upper A-arm. Follow the old brake line path to the right side of the frame. Find the "T" fitting and remove the original rear line and plug it with the 1/4-inch inverted flare plug provided. Next, attach the new line to the rear line with the union provided. In review, the front line of the master cylinder should go to the front brakes and the rear line from the master cylinder should be going to the rear brakes.
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