I keep losing my rear brake fluid!!!

by SonOfTheGrim Mon May 01, 2017 6:33 pm

I'm having some trouble tracking down this problem and it's getting dangerous. Twice now I have lost all the fluid in the rear brake reservoir and consequently lost the rear brakes. I've been under the car several times looking for drips or leaks and found nothing. Everything related to the rear brakes is new except for the hard line going to the rear: Master cylinder, proportioning valve, rear flex line, axle hard lines, and wheel cylinders. I've put two new prop valves and two different slave cylinders on to fix little leaks but am still empty in the reservoir after just two 30 minutes drives. I've also taken the master cylinder back off to check behind it and there isn't a drop. I thought maybe it's leaking into the vacuum booster but even with vacuum it would be wet around the end right?

Am I missing something or does anyone have any insight for a problem like this? It's got me and my dad stumped.

by clanceman427 Mon May 01, 2017 7:56 pm

My metal line rusted thru right at the trans crossmember.

by rosco77 Mon May 01, 2017 7:58 pm

Have you checked the bleeder valves to make sure they're tight? Not trying to say anything bad, but it could happen. I know I've done stupid whoa nelly like that on occasion.

by fasrnur Tue May 02, 2017 12:16 am

That's just weird. You would think that if you keep losing fluid, that it would show signs of a leak some where. When it does finally rear it's ugly head, I'd bet you're going be going..."DOH!" lol

by bigredlaguna Tue May 02, 2017 12:47 am

If the line you haven't replaced is rusty, it could have a million little leaks that wouldn't have enough flow to show a drip pattern. If previous owners haven't kept fresh fluid in the system, it could rust from the inside out. I had the same problem on a previous car of mine.

by thatfnthing Tue May 02, 2017 8:48 am

FWIW, I had a very similar mysterious loss of fluid in the brake line a long time back -- but it only happened when I had to jam hard on the brakes. No signs of fluid anywhere on the underside of the car. Turned out the leak was about half way to the back and the fluid shot directly on to the road, so I never saw it. Mine was rust, but if yours is all new maybe you have an ill-fitting connection acting in a similar fashion...

by SonOfTheGrim Tue May 02, 2017 12:05 pm

I really don't think it's a rusty line or pinhole. I've had my fingers on every inch of every rear brake part and haven't found ANY signs of fluid AT ALL. I've looked and felt at ALL joints, ALL rubber, ALL bleeders, and ALL hard lines. Like I said, the small leak I found was in a brand new wheel cylinder and I've replaced it and still having the problem.

I'll have to lay under the car and get someone to stab the brake hard. Maybe I'll see something that way. I know it'll be a "doh!".

Really dumb question: I am assuming and have traced the lines but am I right that the smaller rear reservoir on the master cylinder IS the rear brakes? Maybe it's the front and I've been looking in the wrong place.

by clanceman427 Tue May 02, 2017 1:32 pm

correct- smaller reservoir is the rear brakes. Much less volume needed for the drum wheel cylinders vs. the disk brake calipers.

by SonOfTheGrim Tue May 02, 2017 2:28 pm

Funny how the reservoir is smaller yet the lines are so much bigger.

by clanceman427 Tue May 02, 2017 3:11 pm

well, remember, coming out of the combination valve, there is an individual line for the front drivers side wheel, and an individual line for the front passenger side wheel. However, one larger metal line for both rear wheels until it splits to individual driver side rear and passenger side rear after the flex hose above the center of the differential housing.

by SonOfTheGrim Mon May 08, 2017 8:06 pm

So after a lot of frustration and redundancy I finally bled the brakes, hopped in, and took the car for a thrashing. Got to the bottom of my hill after a couple hard stabs at the brakes, got out and looked under to see dripping from the axle.
When I got it back in the garage I crawled under it hot and found the unions on top of the differential were both leaking! On top of that the driver side wheel cylinder was leaking from the back too. Apparently there was so much grunge on the diff pumpkin that I couldn't see the new fluid leaking out onto it after it cooled off.
I Teflon taped (I know I know) the threads and wrenched the hard lines down so maybe I got the leaks buttoned up. Haven't had a chance to drive it anywhere but tomorrow I will.
Thanks to everyone for the insight and experience. I'll get back here if the problem continues.

by thatfnthing Tue May 09, 2017 10:33 am

Glad you found it!

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