Quote:
Originally Posted by jocko
The mc's are different. So, a drum/drum mc is not ok for the 72's disk/drum arrangement, as stated above. Disks not standard til 71. Will it "work" - yes, but not correctly. A pre-71 prop valve is mostly just a pressure loss switch that will light up on your dash if you lose a circuit - the proportioning takes place inside a drum/drum mc by the order in which the ports are uncovered when you hit the pedal.
However, just leaving your current master cyl in the mix between your proposed 70 booster and 72 prop valve will potentially not be a direct bolt onto the 70's booster if you add it in. i.e. be aware that, in most cases, the depth of bore in the master cylinder piston is deep in a manual master cyl while it is shallow (and perfectly opposite the booster intermediate pushrod) in a power setup. This could be an issue if you put your current non-power mc onto the 70's booster. If you have this mismatch, you'll need to cobble together about a 1" or more longer intermediate pushrod between the 70 booster and original 72 non-power master cyl.
Personally, I'd buy a new 72 POWER master cylinder and just insert that in your swap and dump the 70 drum/drum master cyl and the original 72 non-power master cyl. What you really need is a correct 72 disk/drum AND power master cylinder.
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I've read that about the rod before, but mine fit perfectly. I don't know if it varies year to year or truck to truck. The new booster I bought fit the non-power master cylinder that was already in my truck. I had the entire assembly from a donor, so I kept all the brackets, turned in the booster for a core, and junked the old master.