Master Cylinders
Our rebuilder machines out the master cylinder and sleeves it with a stainless steel thin walled tube, heat bonded into place. No rough finish on that. They also do slaves the same way. Never has a problem with them, yet.
It is imperative to bench-bleed a new master cylinder before installing it. You will never get it thoroughly bled out of air otherwise. Some say you can adequately bleed them while installed by cracking the brake-line fittings at the MC, but it is not as efficient.
Also, when bleeding the air out of the lines at the wheels ( if using the 2-person pump/ hold/ bleed method), do not depress the brake pedal all the way to the floor...otherwise you run the likely risk of pushing the MC piston too far forward, damaging the seals.
On "vintage" cars many, many people have experienced MC failure only a few miles after an otherwise normal brake service...including bleeding them.
A piece of 2x4 under the pedal will prevent damaging the MC while bleeding the system.
I did a complete bench bleed prior to install (as always). Maybe the MC is defective?
Yeah, if you have thoroughly bled the system, the master cylinder might be defective. Exchanging it should be free so I would try it.

(09-17-2015, 04:09 AM)Mach1FatherFigure Wrote: Our rebuilder machines out the master cylinder and sleeves it with a stainless steel thin walled tube, heat bonded into place. No rough finish on that. They also do slaves the same way. Never has a problem with them, yet.

That sounds like a nice repair. Shops performing that work are not heard of or very rare in the Unite States. Our best option is replace with new a unit.

I have replaced the MC and still the same deal. I must not be getting all the air out is all I can figure. Anyone wanna come take a stab? I pay in beer.
Not trying to be dismissive, but did you follow the basics of proper brake bleeding?
Many try to "shortcut" the process and get poor results.

First off all, since you replaced the MC, it HAS to be properly bench-bled before installation. You say you did that, so good so far.

You MUST bleed all four wheels to insure a good result. Work from the farthest wheel to the closest.
1) passenger rear
2) drivers rear
3) passenger front
4) drivers front

Be sure to use ONLY brand-new brake fluid from a clean and unopened container. DO NOT use any brake fluid that has been opened and sitting on a shelf of somewhere.
Brake Fluid (DOT 3 or 4) is INCREDIBLY hygrscopic...meaning it will absorb moisture right out of the air, even if in a tightly-capped container sitting on a shelf.
Always buy brake fluid in smaller, metal containers. The big plastic bottles may be lower priced, but are more susceptible to contamination, even if brand new on a store shelf.

A vacuum brake-bleeder gun is cheap ( $20-$40), and many auto-parts stores even have free loaners. They work very well.

The idea is not to just remove enough fluid until the bubbles want to replace ALL the fluid in the entire system. Air in brake fluid can be too small to see, but will still give a spongey pedal.
Keep pumping until ALL the fluid has been replaced with fresh. CAUTION: Be sure not to let the master cylinder run dry during pumping or you will have to start all over again. That sucks...
It may take a while a 5-6 small bottles of fluid, but it is well worth it fir a nice firm pedal.

Our brakes are a simple, stone-age and durable design. There is no reason a good, hard pedal can't be achieved.
I dug into everything again last night an found a little leak in one of the rear cylinders. It was just collecting in the drum so I didn't see it dripping. Parts on the way so I should be good. Fix one thing find 10 more.
Hey guys, I was trying to troubleshoot why the brake idiot light won't go out. I bled them a few times and the light would go out then come back on after a few pumps on the brakes. After a while my brakes would occasional sync and i had to do the double pump to stop the car. This would continue even after bleeding the whole system 4 times (rear passenger, driver, front passenger, driver). I ended up replacing the MC and my brakes are now working fine but the only thing left is the brake warning light. I bled the brakes again after change the MC but still stays on. Could this be proportioning valve or distribution block? someone with a 66 mustang told be there is a switch behind the brake pedal. Never heard of it before.
The 66 Mustang owner is correct. On 65 and 66 Mustangs there is a brake light switch that activates the brake lights when the pedal is depressed. The switch is located underneath the dash up in the pedal support assembly area. However, the 65 and 66 models do not have a brake warning/idiot light. Maybe the person was suggesting your idiot light is staying on due to a possible brake pedal height adjustment, not sure.

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