Losing brake fluid

Discovered yesterday that my brake fluid resivor was dry. I haven’t found where the leak is coming from yet but I’m suspecting it’s coming from inside the passenger side drum. Lines look good, no obvious puddles or seepage. I’m about to go outside and take it apart and see how it looks. If it is coming from there is there anything I should be looking for specifically that’s causing the leak?

 

Brake cylinder rubber boot ends. It will be apparent if thats where its leaking from.

 

The drum cylinders, almost certainly what is leaking.

 

Wait...are you losing fluid after you add it or is the fluid level just low? If it's low it's because your pads and shoes are shot. If you add fluid and lose it, that's different.

 

E-brake light was on for a few days. I thought maybe it’s just a sensor for the e brake. Yesterday I learned that it can also signal low fluid. Checked it, it was completely empty. Girlfriend went on a rescue mission to oriellys for fluid. Brought it to me. Topped it off. Drove home behind her a few miles. Checked it again and the level definitely dropped a little.

 

Welp..

 

Wasn’t expecting that. Thick black grease everywhere. Inside of the drum, brake assembly. So that would be an axel seal?

 

Does it smell like diff fluid? Pretty distinct smell. Could be brake fluid and brake dust together

 

I think so. It’s pretty pungent. But it’s a lot thicker than diff fluid. This is thick grease.

 

That just looks like good moly.

 

You may have run it dry and may need to bleed the system. Brake fluid flows and you should see it on the inside of the wheels if it's leaking at the cylinders in the rear. Bleed it, check it after a few miles.

 

That's certainly not correct, I suppose that could be a mix of brake dust and leaking diff fluid. Doesn't really look like it from the pic though.

 

You'd see brake fluid running outta the drum if the wheel cylinder failed. You're looking at a blown axle seal.

If you're low on brake fluid and you can't find the puddle, it'll be in the brake booster.

 

Brake booster was my next suspect.

 

You may be correct but blown axle seals don't cause brake fluid loss. Not sure he is actually losing fluid though.

 

Ok so I had the mysterious evaporating brake fluid issue for a long while. I replaced the rear cylinders and, because why not, the brake calipers. They were ok just blown piston seals, no leaks but uneven pad wear so a sticky piston was the likely cause.

Anywho... so it turns out that the leak was from the back of the master cylinder, and the fluid was going into the booster. Pedal went completely cock stiff one day causing me to do more troubleshooting. Booster had quite a bit of fluid in it. Soooo your master cylinder may have poor seals and voila. Same issue as me. You can pull the bung where the vacuum line connects to the booster and take a string or something and drop in in and see if it comes out wet. It should not be wet

Edit: Also check your proportioning valve on the rear axle if you have one. That is a potential leak point that is tied into your brake system.

Edit Edit: I just saw Andy's post (#13). Yup but here's my story as well.

 

If brake fluid drops, and there is no leak, trust me it's going somewhere.

 

So when pads wear the fluid level stays the same? Nah.

 

Oh it’s going somewhere alright. Not sure what the booster/master cylinder lifespan is suppose to be on these things but the truck is at 315,000 miles with most likely the original booster/MC

 

Keep watching the fluid see if it keeps dropping.
I think you were just low from pads wearing. I’d make sure you’re consistently losing fluid before jumping to the more complicated and expensive causes.