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Discussion Starter · #1 ·
For the Rat/Shoestring project I'd like to ditch the F-150 power brake unit because it is so huge and difficult to mount inconspicuously. I happen to have a much smaller PB unit from an old Mazda pickup truck. In doing a forum search Willys noted that one should only use a Master or PB unit from a vehicle of roughly the same weight and that it was preferable to use the entire system if possible (rather than mixing and matching components.)

Regarding weight, I am guessing that the rat, even with a 351 & C6, will be pretty close to the weight of the Mazda. I would, however, like to use the combo valve from the F-150 since it would have the correct metering and proportioning already dialed in - and it might be different than the Mazda combo valve.

Anyone see a huge problem with using the Mazda PB unit and/or the F-150 combo valve? And just to be clear, the Mazda had discs in front and drums in the read just like the F-150.

Dewey
 

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The master cylinder bore size and volume should be matched to the caliper and wheel cylinders to ensure the correct pressure and volume of the brake fluid.

The booster is for power assist. A smaller diameter booster will give less assist. There are 7" dual diaphragm boosters available for street rods that should work fine. The Mazda booster may work OK due to the weight of the vehicle you are building. I'd try it first.

The Ford proportioning valve will probably not work as you expect due to the different (lower) weight front and rear of the vehicle you are building as compared to the original F-150. An aftermarket adjustable proportioning valve in the rear brake line would be my suggestion.

If you are mounting the master cylinder below the floor you will also need a 2 p.s.i. residual valve for the front disc brakes and a 10 p.s.i. residual valve for the rear drum brakes.
 

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Discussion Starter · #3 ·
I was a little leery about using the F-150 combo valve anyhow so I'd already factored the residual valves and the proportioning valve into my budget. Thought I might try the stock combo valve first but now I think I'll just go with your suggestion.
 
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