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The 69 Bonneville has a single chamber master cylinder, I want to upgrade to a dual or separate chamber and a brake booster. I priced a few from the most of the parts sources; Summit, Auto Zone and Rock Auto. I friend " Also and Shade Tree Mechanic" suggest  pulling one from a junkyard. Has anyone actually done this with success? In theory it may work, but I would like to talk to someone that has done it.

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Junkyard is possible, but in the case of your '69 Bonneville, whatever used parts you end up with, you would still have to rebuild the master cylinder and at least pressure test the boaster to insure its safety and reliability. In this particular situation, for my own piece of mind, I would prefer to buy a new, off-the-shelf kit instead.

Also, are you going to consider a front disc brake conversion (that's assuming you are running front drums that is)? Now would be the time to do that as well.

Edited by Frosty
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It's standard practice to treat any boneyard part over 10 years old as a rebuildable core. Where brakes are concerned, doubly so. I've heard several people tell of their good MCs springing leaks just from trying to bleed the system. For years, the pedal never traveled more than the 2 inches necessary to stop the car. While bleeding, the pedal traveled further, and a worn lip in the MC nicked a seal. Better to assume the worst, pull it apart, inspect it thoroughly and rebuild as necessary. In the end, you wind up with a part you know is done right ... instead of hoping some 8 year old in China wasn't having a bad day.


I definitely agree with the idea of a dual system. Many new cars today are back to a single and bank on the ABS system to save you in case of a line leak.  Stupid idea. BTW, while you're there, may as well replace those old rotted steel lines with new Nicopp lines. Best investment I ever made. Bends easier than steel, doesn't kink, and NEVER rusts. Why it's not standard on every car today (it is in Europe) I can't fathom.

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Hi,  Mike,

     I was surprised to read that your 69 Bonneville has a single chamber master cylinder - I thought all North American cars went to dual chamber systems in '67. I checked Rock Auto's website and see only dual chamber master cylinders listed for 69 Bonnevilles. (I had a 69 Grande Parisienne, Canada's answer to the Bonnevile. Really nice car.) I wonder if there is a proportioner valve, or a combination valve, on your car near the m/cyl (usually brass, usually within 2 feet of the m/cyl). This would suggest that the car originally had a dual system. If nothing is there like that, and you go to the dual m/cyl,  you're going to need such a valve - it splits the two lines from the m/cyl into four for the wheel cylinders (and calipers, if the car has front disc brakes.) Some earlier cars/light trucks had one output line from the valve to the rear, and one to each front wheel. I couldn't find one on Rock Auto.

     The prices I saw for the m/cyls were pretty low, so I'd go with a new or rebuilt one. The junk yard route has the advantage of being able to get the push rod between the booster and the m/cyl, and the proportioner valve, if you should need it. Your single chamber m/cyl leaves me unsure what to expect when you take it off of the booster, as to whether the push rod is the same as for the dual m/cyl.  You do have power brakes, right?

     There's another possibility, if you're feeling venturesome. The m/cyl for an 89 Buick Electra, for instance, has four outlets - one for each wheel. I think it may very well bolt up to your booster. Going that route simplifies plumbing a bit, especially if you already have front disc brakes. Otherwise, Speedway Motors, and very likely others, sell residual pressure valves, which they recommend. I don't have them, though, in the split/dual brake system on my 53 Catalina (all drum brakes) and they (the brakes) work well.

     Hope this has been food for thought, or at least interesting.


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