New Master Cylinder leaking from reservoir to reservoir?

[73k5blazer]

Anyone every heard of this?
It is not for my k5, but my 1970 Impala with front disc/rear drum power brakes.

I bought a new master, bench bleed it, bleed the 4 corners, all seemed fine, until I noticed a leak, coming out the top of the master. I open it up and fluid was nearly empty in the back reservoir (front brakes) and full to the brim in the front reservoir. Ok, WTF, I remove some fluid from the front, fill the back to 3/4 full put the cover on. About 50-60 brake presses later, back reservoir is empty and front is full and coming out from under the cap under pressure when you hit the brakes. Which is scary on that car, the OEM iron manifold (car is all original 63kmiles) is right there and is extremely hot of course.

The only thing I could think of is the master I got has to be put together wrong and allowing fluid to flow from front to rear. The reservoirs are of course completely separated in the cast iron reservoir compartments for safety, with literally no path between them, except of course the bore below but the bore piston is supposed to prevent that. The master was brand new, not a reman., not a cheapy either.

I replaced the master with another new one (different brand) and that one works fine, thankfully. But it was kinda a shot in the dark, WTF is causing this?

I just never heard of a new master failing or rather, simply put together wrong. I guess, it can and does happen.

 

[diesel4me]

Never saw one transfer fluid from one chamber to the other (yet!)--but I guess anything is possible....I'd assme it would have to be an internal problem,possibly caused by improper assembly?..

I've had plenty of master cylinders leak the fluid from the rear chamber into the booster though ,and get burned in the engine,(and eventually ruins the diaphram in the booster) --or drain onto the floor via the drain hole in the casting--including the one on my '82 K2500,which I have to refill every time before I drive it,or before it goes empty,which can take anywhere from 2 weeks to a month...

I've been meaning to replace it,but I needed 50 bucks to "buy" another one so Autozone can refund the money when the replacement one arrives,its a "special order item" now..oh the joys of owning old fossils....for now I've been keeping it full ,and it has hydroboost,so no booster to get ruined..
I bet I've spent 100 bucks on brake fluid since it started leaking last fall.....enough to buy a rebuilt 3-4 times already..

 

[kennyw]

Just sounds like it was assembled with bad parts or the wrong parts all together. Time to exchange it under warranty.

 

[Rugger187]

I've had that happen before. What usually happens is the place that rebuilds them just replaces the seals. My piston was eroded and fluid went past the seals that way.
I got sick of all that and swapped in a hydroboost and a brand new master for disc disc.
I am running it in a 65 Bel Air with a 454 and front and rear discs. I swapped in a ford 9" with 3.90 gears and a Detroit locker. Now that beast stops better than anything I own. Including the wife's 2013 Altima.