Losing brake fluid

Pull back the dust boots on the wheel cylinders and see if there's brake fluid behind them. Or just squeeze and see if brake fluid comes out

At 300k plus miles you could have blown wheels cylinders, leaking axle seals and a bad master cylinder. Keep looking

 

Are we sure the master cylinder reservoir could go bone dry from brake pads wearing? My pads have very little life left (they're on my to do list before elk season), and my brake fluid level is still plumb full. I'm not really a believer that the fluid level would move significantly from pads wearing.

 

That’s what I’m thinking too. Both my pads and shoes still have decent material on them. And if they were low, I don’t think it would drain the resivor that much. I mean it was empty empty.

 

Yea your booster seems a much more likely culprit.

Pad wear theory ain't ringing true to me

 

That's just the easiest answer for the light coming on from the fluid dropping some. I don't remember how much the fluid had dropped last time that happened to me, but I'm pretty sure the fluid float was sitting on the reservoir floor.

 

I’ve seen cars with full metal to metal contact with the backing pads and rotors and not drop the reservoir low enough to have the light turn on. If it did it might have saved the owner some money haha

 

Since you say all the lines are good, if the reservoir level has dropped to empty and you don't see a fluid stain on the inside of any of the wheels, I'm joining the bad booster voters.

 

Yeah most likely your master is leaking into the booster....

so now the fix is both a new master and a rebuilt booster.

As far as the rear grease......only source would be rear bearings....
if those seals are leaking it would be cascading out of the axle.

maybe its just starting to fail and just a coincidence that the master is failing too.

 

Axle seals can leak slowly too. I had a slow leak when one of mine went. Makes a bunch of gunk in the drum from brake dust.

 

Your worst mistake would be to run the diff dry.....
those salvage start at $1200 to $1500.......if you can even fine yours...

So I'd be tempted to pop that plug and top that off to be safe.

 

Well it’s in the shop now. I don’t have the time, space, or tools to do the axel seals myself and I’m having them take care of the brake fluid issue too. I appreciate all the helpful replies and I’ll update this thread when I get it back.

 

I was curious why that gunk was so thick. I don’t recall grease like that being apart of the axel assembly unless I’m mistaken.
Is that thick black grease just gear oil mixed with brake dust??

 

Means you've probably been stopping using almost all front brake....
that may of been what did in the master...

5vz w/ 4X4 is a pretty heavy vehicle.....

 

Let us know what they find and how much...

It took over a $100 in specialty tools.....dozen videos......6mos planning....and all day to do my rear seals...


The booster is a real concern......I've seen them as cheap as $200, and some that were almost $900.....

 

Any brake fluid that gets past the brake cylinder dust boots (the part you see when the drum is removed) first gets past the seals that are inside the cylinder and on the two pistons (the parts you don't see until you disassemble the cylinder).

 

Clean or replace the rear diff breather too!

 

What’s crackin?
How is that diff breather plug looking?

 

Exactly.

 

Yeah the grease in the bearings won't get outside of them unless it's flushed out by gear oil from the diff.

IDK about pad / shoe wear running the brake fluid reservoir dry - that would be a poorly designed system to not have a safety factor built in. Again lots of people will get down to metal on metal on their brakes and not run the reservoir dry and have the light come on - just google some videos of mechanics making jokes about customer mistakes they find.

 

I have never seen an OE brake system where the low fluid warning light won't go on before the pads are toast. People get the light and add fluid 99% of the time I would assume. You should only replace brake fluid, never add it.