Master cylinder pressurizing

[Ren]

'67 F250, 2wd, automatic, stock, factory power brakes, all drums.

I've owned this truck 8 years. Among the many things I did when I got this truck was replace the master cylinder with a unit from Oreillys. I forget why. Probably just for good measure. The new master cylinder and brakes worked well, but there was always a slight weeping of brake fluid from around the reservoir cap. I tried many things to solve this, but never succeeded. The master cylinder rusted terribly because of this.

8 years and many miles later, I replace that m/c under warranty with another one from Oreillys. Before this, the brakes worked excellent, but I finally got tired of the cap leaking.

After bench bleeding with a syringe, I installed the replacement m/c, and after bleeding the lines, I could not get any usable brake pedal no matter how much I bled the lines. Pedal went to the floor with very little effort, barely functions the brakes, and does something I've never seen before: the more I press the brake pedal, the more pressurized the m/c reservoir becomes that I can actually hear it hissing from the cap. I removed the pistons from the m/c, and could see no obvious defects. Reassembled, tried again, same problem.

I returned the m/c to Oreillys, and bought a new one from Rockauto. New, made in China. Same exact problem as before. No pedal, pressurized m/c reservoir, leaking from cap.

I bought a new m/c from Napa. Made in China. This m/c started out very promising. I was able to get pedal in short order. Victory at last! Drove 5 miles, brake pedal goes to the floor, pressurized reservoir, fluid leaking from cap. This time I had an automotive savvy friend help me out. We crawled around looking for leaks in lines and fittings, Removed and inspected all the wheel cylinders, scratched our heads and tried unlikely things. And he is also without an explanation.

This is not my first rodeo with brakes. I am not a professional, but over many years I have successfully replaced all sorts of brake parts on many different vehicles and they all ended with success in an appropriate amount of time...until now.

The only thing I can think of is that I am somehow ruining a seal while bench bleeding with a syringe.

Any feedback or suggestions are appreciated.

[cep62]

I would make some plugs (out old line and fittings ) and cap off the mc
If you still don't have a pedal it's the mc if you get a good pedal it's somewhere else.

When bench bleeding I've always used the supplied kit and bled it back to the reservoir.
its been awhile, do they still supply a bleed kit ?

[Ren]

I haven't seen a bleed kit come with a m/c in many years.

I tried your suggestion of blocking off the ports with the last m/c. The pedal went to the floor and pressurized the reservoir with little effort.
But getting three bad m/c's in a row is such a long-shot that I am wondering if I am somehow doing something wrong.

I found a nos, made in USA m/c on ebay. I hope this solves my problem.

[Ren]

Update:

Installed a nos, USA made m/c. I no longer have any brake problems.

The point of this is to warn and illustrate that I got three defective, new, made in China m/c's in a row. They were from Oreilly's, Rockauto, and NAPA.

[pcmdub]

Ren,

Good news. Here is my GUESS...

Regardless of the MC origin, I think the issue is that new/rebuilt Master Cylinders do not have Residual Pressure Valves. The MCs are for disc brakes and more universal. So they don't install the 10lb residual pressure valves. The NOS MC does/should have them installed. You can use a newer MC if you just buy the residual pressure valves separately and install them inline. Also, you should always bench bleed the MC if it is new. All of mine come with a bench bleed kit but you can just use some cheap hoses.

[basketcase0302]

="Ren"Update:

Installed a nos, USA made m/c. I no longer have any brake problems.

The point of this is to warn and illustrate that I got three defective, new, made in China m/c's in a row. They were from Oreilly's, Rockauto, and NAPA.

Everyone needs to read this thread!

And like CEP62 said I always bench bleed with the fittings, (it's easy enough to fab some lines for the next bleed with a few flare fittings and two short pieces of tubing).

I had a storage unit many years ago in South Florida, (put the 10" of lift into my old style Bronco there).

The old Cuban guy across from me had a "side hustle" rebuilding MC's/alternators/and starters.

He got paid "piecework" folks, (not by the hour but by the volume he cranked out)!
So yes ALWAYS question the QA or quality or any rebuild, (especially if it is of Jackie Chan origin)!
I just went through 3 new fuel pumps on my Escape last year before having to go with Ford OEM that corrected the issue of the fuel pump not communicating with the ECM.