Replacing the master cylinder - Impala Tech
Brakes & Suspension Conversion Questions & more

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post #1 of 3 (permalink) Old 09-20-2017, 04:07 PM Thread Starter
Join Date: Aug 2016
Posts: 44
Replacing the master cylinder

My dad and I have run across an issue with our brakes and we need to replace the master cylinder and booster.
1) Do we need to get new a master cylinder and booster if we plan on converting to disc in the front later? (currently running 4 drums)
2) Is there a particular size diaphragm that is best for a full size?
3) Are most master cylinders and boosters universal?

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post #2 of 3 (permalink) Old 09-20-2017, 06:33 PM
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The booster is universal. It is a can that has an eleven inch rubber diaphragm inside that actually pushes on the master cylinder piston for you. You are opening a valve to let air into the booster and the manifold vacuum pulls the b\rakes on. This works well so long as you do not have a cam that goes rumppity-rump. If you do, you no longer have manifold vacuum, and hence you no longer have power assist brakes.

As to the master cylinder there is not only a different size piston inside the casting (which is the same as all of the others so they interchange) between the manual brake and the power assist brake the actuator rods are different as are the mounting point on the brake pedal. Here is a YouTube showing different sizes and there are tens of different sites on the net explaining the difference in brake feel and performance based upon piston bore size.

On older cars they punched two holes and mounted the rod to a different hole depending upon the time of brake application. To save on tool sharpening costs the factory went to two different brake pedals once power assist became standard equipment (and you had to order manual brakes as an option).

Here are two 1977 Nova brake pedals.

The one on the left is for power assist the one on the right is for a manual brake. Note the difference in the distance to the rod attachment point that changes the angle of the rod and the length of the stroke.

Back of master Cylinder piston has two different seats to accept two different length rods. On the left is the long 4 inch push rod. On the right is the 2.625 inch short push rod seat.

These are some of the tiny differences most do not know about, and that cause headaches and poor brake performance when not addressed. This is why I advise buying a COMPLETE kit for a vendor that has everything from hardware to sealant. Brakes are important if mot for you then for me, we might meet by accident otherwise.

Big Dave
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post #3 of 3 (permalink) Old 09-21-2017, 10:09 PM
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Join Date: Jul 2011
Location: Los Angeles
Posts: 118
If converting to front disc and retaining rear drum, the brake lines require different pressure. In this case when you get a new master cylinder, buy one with a proportioning valve to control front and back pressure independently.
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