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upgrading the brake master cylinder on my 79 620


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Yes........ it should. You are going up two sizes you know? The pedal will feel much firmer. Also the zx master is set for use with a rear disc brake set up. This will provide much more fluid to the rears and may cause the backs to lock up long before the fronts. Rear wheel lock up will cause the rear end to try and 'come around' on you with massive over steer. This is even worse in the rain. Bad enough in a straight line but very dangerous on a turn as you can't always steer into the slide.

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That's no upgrade. What are you trying to accomplish, perhaps we can suggest a solution.


I have been wanting to upgrade the brakes on my truck for a while now without throwing calipers and rotors at it. I thought that with a larger master it would push more fluid easier making more pressure and stopping better while the master cylinder isn't working as hard. What do you mean its not an upgrade? what would you suggest?

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Unfortunately it doesn't work that way. It has everything to do with mechanical advantage like a small jack with lots of pumps can lift a heavy car, but you can't lift it all the way up with one.


If your master was 1/2 square inch area and you push down with 100 pounds of leg pressure you will exert 200 PSI of hydraulic pressure in the system.


If you instal a 1 square inch area master and push down with the same 100 pounds of leg pressure you will have only 100 PSI in the system. That's half the pressure to work the brakes. You will have to press down harder (twice as hard) to get the same brake performance.


Going to a smaller master will make the pressure higher and the foot effort less but just like jacking the car up the peddle will have to travel further, closer to the floor and feel spongy.


Going to a larger caliper will require more volume of fluid, so again the peddle travels further and closer to the floor and may feel spongy.


A larger caliper will usually require a larger master to supply the volume. A larger master will again usually need a larger brake booster to reduce the effort. It's a fact of life that you can't get something for nothing.

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