Master cylinder question.

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Fredensborg

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So I got a quick master cylinder from Napa to keep my car driveable while I try to rebuild my old one. But the 2 break lines that go into it are different sizes, and they are reversed on the new one…is this ok? 182FC2E6-09C0-4C8A-B916-DFCD8C99CA4D.jpeg On the old one, the larger head goes to the front, smaller reservoir.3C5490A0-02D8-4AFA-BB5E-94DB9BF392F3.jpeg on the new one, the bigger hole goes to the rear larger reservoir.
 

Hemikiller

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Fitting sizes changed on 7/1/72 with a bore size change. I suspect you have a 71/72 brake setup on your car.

Can you measure the thread sizes on your existing fittings or MC? The 71/72 master was 1" bore, 1/2-20 forward, 3/8-24 rearward, part # 10-1378. The 7/1/72 & later MC was 15/16 bore, 3/8-24 forward, 7/16-24 rearward. Part # 10-1526.
 

Fredensborg

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I’ll check when I get home. I was hoping Steve would have the O’Riley part number so I could get one that I know will work for now. I’m planning on rebuilding my old one.
 

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Unfortunately , my receipt is in AZ with my car.

wait, I found my hpnote:

BrakeBest Select Brake Master Cylinder - New! Part # NMC90088

the way they deal with the front fitting is to include an adapter to fit either size fittings. That’s my assertion since I haven’t mounted it yet, but I did dry fit it.

Both of the Autozone MC didn’t fit my car. The front fitting opening was either too small or large.
 
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The larger reservoir is for the outlet port that feeds the front disc brakes. The smaller reservoir feeds the outlet that feeds the rear drum brakes (assuming you have not installed rear disc brakes). That is because the disc brake calipers use a larger volume of brake fluid than the rear drum brake wheel cylinders. If your ports are not going to the correct brake units (front and rear) you will run into several complications. First, the drum brake outlet port has under its brass seat a small rubber Flapper Valve that is designed to retain a certain amount of pressure in the brake lines feeding the wheel cylinders (Residual Pressure Check Valve - RPCV). This is not something you want to be in the hydraulic circuit for any disc brake system, as it will maintain a steady amount of pressure on the brake pads which will lead to less vehicle performance as the disc pads will be dragging all the time, overheated pads and/or rotors (rotor warpage), and excessive pad wear.

I would definitely swap that MC out for one that has the larger and smaller reservoir in the correct position (what the car had originally). And, just to be safe about it, before installing the new MC look into the small hydraulic fluid opening of the two outlet ports to make certain the little rubber RPCV Flapper Valve is visible for the rear brake outlet port only. You will need a small flashlight to be able to see in there. If the outlet port for the larger reservoir has a RPCV Flapper Valve in it, that MC is for an all-drum brake vehicle, not a front disc brake vehicle.

Also, you definitely want that RPCV Flapper Valve to be present on the outlet port that feeds the drum brake wheel cylinders - it is critical it be present, otherwise when the brake fluid cools the wheel cylinder cup seals will suck air into the hydraulic system as the fluid volume decreases. We do not want air in the hydraulic system, ever. A secondary reason for the RPCV for drum brakes is to hold pressure against the brake return springs in order to reduce the amount of pedal travel to actuate the brake wheel cylinders enough t cause the brake shoes/linings to be pressed out toeard the brake drums.

 
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Just went through this on my car with Oreiley auto parts…… the rebuilt I received from them had a compromised internal cylinder surface. I only discovered this after installing it, found it leaking and then putting in a rebuild kit and discovering it still leaked. This proved to be a MAJOR PIA! I hate brake fluid leaks and what it does to any painted surface if left undiscovered.

If your master cyl is original it’s worth hanging onto and rebuilding.

While replacing calipers, my local Oreiley parts guy told me he can get actual FOMOCO rebuilds for a few dollars more instead off an offshore replacement casting part….. I did it and was impressed. The same MAY be true of the master cyl. A true repo should have a dot in the casting above the rear brake line fluid outlet to be “original”…… if your still has it, I’d hold onto that one and rebuild it later.
 

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