Last week my brakes started acting up again and the pedal started sinking to the floor for any brake pressure and I have to “pump” them to get pressure back at normal pedal press depth. So I get a rebuilt NAPA master cylinder yesterday and put it in this morning thinking that’s all I needed. I bled them this morning and now its worse. My brakes stop normal at ˝ pedal presses, But at 5/8 to full pedal press I get a “pop” sound and I have zero brakes. Scary! This is the 2<SUP>nd</SUP> time I’ve replace the master cylinder in a year.


Further examination is that the "pop" is actually something giving inside the master cylinder and the fluid starts to fill up the plastic reservoir and almost overflows. The brake lines aren't getting the fluid. My brakes aren't hard so i think the brake vacuum booster is working fine.



My questions are:
Is this mastercylinder toast also?
Should I buy brand new instead of remanufactured Master Cylinders?
Why would the fluid over flow the reservoir?
I'm using Prestone syntetic Dot3 fluid I hope it's not ruining my Master Cylinder valves.

 

Make sure to bench bled the master cylinder thorougly before you install it on your bronco.

 

Thanks much. I will look up "bench bled", do it and report back. Much appreciated!!!!

 

Here you go check out this site, it cover brakes very well. http://www.classicperform.com/TechBo...oubleshoot.htm

Here is the master bleeding section: http://www.classicperform.com/TechBook/BrakeBleed.htm

 

just curious. why would there be any reason a air filled MC shoot fluid back up inside the reservoir causing it to overflow? Wouldn't it just be "spongy"?

 

I'd say you got some big *** in there thats why ...Not small bubbles of air and the fluid can not break the air bubble ....They are a dam tricky lil thing to do especially alone I know I had a hell of a time with mine in my old truck years ago ...Then got a friend over to help and it went way better ...TR

 

It was a bad Master Cylinder. What luck :-/ I guess next time I will just buy a new one, the reservoir is getting stress fatigue from being pryed off a few too many times anyways. Thanks for all the help everyone.

 

Use an old push rod or something that is long and has a round point, when bench bleeding your brake master cylinder. Make the master cylinder reservoir is about 3/4 full of clean brake fluid, and the two rubber hose coming out of the the holes on master cylinder are submersed in the brake fluid during the whole bench bled process. Push in the piston on master cylinder till it bottoms out, there should be no air coming out of the hoses when you have completed bench bleed process. Process should take about 30 minutes.