Ok My 64 dodge has a single fruit jar master cylinder I want to know if I can replace the fruit jar single/ with a dual DISC/ DRUM master cylinder , and keep my drum brake system (for right now) ? The reason I want to do this is I plan to install front disc's at a later time. This way I would only have to buy the disc brake master cylinder. Then convert to front disc's later ?
IF YOU HAVE DRUMS USE DRUM MASTER CYLINDERThats the rules I belive the disk master has some type of different porpotining valve built in.The bore of the drum brakes must be matched to the master cyl
Yes, you can do this. The main difference in a drum vs disc brake MC is the Residual Pressure Valve is built into drum brake MC's, with the appropriate pressure of about 10 psi, whereas the disc system needs only about 2 psi, if any at all, and may not have a RPV. One can be installed inline to solve that issue. The bore size is a consideration, but mostly based on whether you use a power booster or not. The only down side to a bit larger MC bore on a non-power system is higher pedal pressure. Also, the pedal arm pivot to MC pushrod position is usually different to partially compensate for that.
There is no good reason not to use a disc/drum master on a drum/drum system. Your older drum brakes do require residuals however, so you will have to check for a rear drum brake internal residual on the master you use, and if none present, install a 10 lb inline rpv, also adding one to the fronts from the disc circuit of the master.
I will be staying with manual (non power ) brakes. My current drum drake system is manual (non power) The current fruit jar single MC bore is 1.00 In 1971 dodge vans had a disc/ drum manual master cylinder (non power) with a bore size of 1.03125" With a slightly larger bore (.03125") might require a little more leg pressure. But the advantage of the front discs should stop better, and not need as much effort. (might be wrong about that )
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