Quote:
Originally Posted by Andy4639
The master cylinder I bought was for a 72 C 10 with disc brakes & drums. It was said it was for any Chevy truck for that year. Manual are power, c-10, c-20, c-30. 
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Whoever told you that was wrong twice. Manual and Power are different MCs as has been discussed above, but an equally important difference exists between C10 vs C20/30. In other words, a 71-72 C10 mc is not the same as a 71-72 C20/30 mc. They are biased and plumbed differently. Front reservoir on a C10 mc is for the front disks, whereas the front reservoir of a C20/30 mc is for the rear drums (note the smaller 3/16" lines are always for disks and larger 1/4" lines are always for drums). 1st pic is C10, 2nd pic is C20 (same as C30). The distribution block is the same - i.e. front is front, rear is rear. To go from a 68 C30 drum system to a 72 C20/30 disk/drum arrangement, you'd technically need a 71-72 C20/30 mc (manual or power, your choice - but note that by 72, power brakes became mandatory on C20 and above), a C10-30 distrib block, and would need to replumb lines appropriately from the mc to the block (front res to rear of block/rear res to front of block) and then replace the remainder of the existing front drum system's 1/4" lines with 3/16" lines for the 71-72 disks. Not everyone does a line reduction in the front, and the system still works of course - but if you want it to stop exactly like a 71-72 C20/30, the front lines should technically be 3/16".