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Old 10-13-2013, 02:31 PM
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Brake Master cyl and wheel cylinder size questions

I am building a 1965 Falcon Ranchero. I have installed a Mustang II suspension,steering,brake system up front, manufactured by TCI. I'm running a 302 w/AOD tranny. Original 8 inch diff with drum brakes in the rear. Everything in the brake system is new, including brake lines and fittings. TCI informs me that their system has 1980 Ford Granada rotors and 1985 Camaro calipers. I opted for a 1979 Mustang vacuum assist brake booster and '79 Mustang Master Cylinder. I originally installed a combination valve from So-Cal. I was unable to get a hard pedal with this setup before starting engine and activating the booster. When the booster is "on" the pedal goes to the floor with very little if any resistance. I made some calls to TCI and to Baer Brakes in Phoenix and they informed me that they do not use a combination valve; instead they use a adjustable proportioning valve in the rear brake line with a 10 psi residual valve. So, I removed the combo valve and installed a Wilwood adjustable prop valve. Same result. So, I got another new master cylinder (same type) and had the same result. So, I got another booster (same type) and same result. I get a semi-hard brake about 2 inches from the floor without boost and pedal to the floor with the engine running and booster operating. Made some more calls and they advised that I needed a 7/8 inch master cylinder bore as minimum. Called NAPA and they advised that the '79 'Stang m. c. was a 7/8" bore. Was told I should get a larger bore.
Did some reading in my brake manuals and found that a larger bore would provide more fluid and smaller bore would provide more pressure. Now I am lost! Does anybody know what combination may resolve my problem? No brakes is No FUN! Pressing on with AC installation and exterior trim. This little car is ready to rock and I'm afraid to let it roll. Thanks Bill Winnett

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Old 10-13-2013, 03:28 PM
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I'm unfamiliar with the '79 Mustang master, but assume it is a dual reservoir unit. Plumb the larger reservoir to the disc brakes and the other to the drums. Use a 10 lb residual valve in the rear line, along with an adjustable proportioning valve. Adjust the rear brakes up against the drums. Then, top off the master and bleed the brakes, beginning with the passenger rear, then the driver rear, then the passenger front and finally the driver front. Have your buddy standing by with a can of brake fluid so he can continuously top off the master as you bleed the brakes.

In my opinion, you would only need a 7/8" master if you were running manual disc/disc. Disc/drum with power assist should use a 1" or 1 1/8" master, again, in my opinion.
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Old 10-14-2013, 09:50 AM
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Everything Tech said is correct. Did you bench-bleed the master before installing it? Have you checked both pushrod lengths?

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Old 10-14-2013, 12:28 PM
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Power Brakes with Disc Front and Drum Rear

Trouble Shooting your system.

No Pedal: Defective Master Cylinder. Booster Pin in MC is too Short. Air in System, Low Drag Calipers, Caliper Bleed Screw is not facing up at 12 O'clock position.

Spongy Pedal: Defective Master. No Residual valve to rear drums (10 lb residual valve required). Air in system. Small Master Cylinder Bore Diameter (with this type of Disc/Drum set up you should have a 1" Minimum MC bore). Drums too large. Incorrectly arched shoes.

Since you changed out your Proportioning valve to a manual one, no need to tell you that the proportioning valve should be BELOW the MC, not above it.
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