I have two trucks a 55 and a 65.Both are disc front drum rear brake setups.The 55 has a power unit and the 65 does not.The only difference that I notice is that you have to use a little more foot pressure without the power assist.On my 55 I bought a dual power master cyl. from Chevy Duty,( different name now) that mounts in the same position as the original Master cyl.on the frame rail under the floor.Everything was included and it was an easy install.Of course it was a little pricey,but sometimes its better to pay a little more and get something that fits right up.Just my 2 cts.worth.If you go with the powetr unit,make sure you hook up the vacuum line directly to the intake manifold.

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