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brand new master cylinder with rusty fluid

shock88

shock88

Electric Pole Dancer
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hey guys, i put this new master on about a year ago, all new fluid etc. since then the trucks been in the garage, zero miles. what happened? It appears to be rust that was forming at the bottom, very soft chunks of it. I’ve also only had plastic reservoirs. This is my first all iron one. I was initially swapping this new unit out again because it started to weep between the master/booster. anyone have insight? IMG_6929.jpeg
 
lots of the cast iron don't seal great on the cap . i always need to top mine off 2-3 times a year and no leaks .

do NOT try and use that again . them chunks and crap will trash the system .

hate to say it but time for another new one . what are you running ? hydroboost or vac brakes .
 
sweetk30 said:
lots of the cast iron don't seal great on the cap . i always need to top mine off 2-3 times a year and no leaks .

do NOT try and use that again . them chunks and crap will trash the system .

hate to say it but time for another new one . what are you running ? hydroboost or vac brakes .
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ya i already planned on swapping it out due to the small weep out of the back seal but when i popped the cap i was really confused, i’ve never seen a new master and fluid go bad so fast. I’m running a hydroboost set up. i did read many years ago about a 3500HD master possibly working, i just need a bigger bore due to my 1 tons with disc all around.
 
sweetk30 said:
should work . i never did it yet . but i have 3 3500-hd trucks here i should try it some time .
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i’ll bring this one to the part store, see how some different bigger bore masters measure up and report back
 
shock88 said:
ya i already planned on swapping it out due to the small weep out of the back seal but when i popped the cap i was really confused, i’ve never seen a new master and fluid go bad so fast. I’m running a hydroboost set up. i did read many years ago about a 3500HD master possibly working, i just need a bigger bore due to my 1 tons with disc all around.
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As @Fastereddie mentioned, the brake fluid easily absorbs moisture from the air.
When you drive it often it's getting hot and moisture evaporates, but when it sits long it accumulates and that's the result
 
So i found one that seems to work, i dont have any real world review for you guys because i haven’t started my motor yet, but physical fitment and the line ends threads match up so it seems like a good upgrade from factory. It has a 1.25” bore, compared to a 1.125” bore from a typical half ton. the one i removed had a 1.31” bore. But rusted to all hell like you guys saw. A good easy swap for what you get. It is Napa #M2904. IMG_6988.jpeg
 
sweetk30 said:
The aluminum / plastic you got . Was it for a mid 90's 1500 6.5 diesel combo ?
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this is off napas website, but the list is longer if you scroll through, it appears to be a popular master for the beginning of the OBS years. I’m not sure of the engine applications though. IMG_6990.jpeg
 
To match up to the bore of the hydroboost in a lot of them it has to be 6.5 equipped to have the hydroboost .

Be interesting to see its performance letcus know .

I used one on my 2wd build yet to run it .
 
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