Very slow brake fluid leak

So I marked and dated the level of fluid (full line) in the reservoir. It has taken 4 to 5 months to leak down to the "Min" line. I cannot for the life of me find the leak, or evidence of a leak anywhere under my truck. The calipers are new re-manufactured units from Toyota, I have newish Toyota cylinders in the drums. I have never touched the hardlines anywhere except for the front brakes. I have no evidence of fluid leaking anywhere...

WTF (Where the F) is the fluid going. I have no leaks in the master cylinder that I can see either...

Thoughts?

 

How are your brake pads? When the pads start to wear down low the fluid will begin to drop a bit

Although going from 'full' to the 'low' mark is pretty extreme...it's gotta be leaking slowly from somewhere. Is the cap on the reservoir completely sealing it?

 

Internal leak in the booster?

Look up into the pedal box to see if its leaking through the cab.

 

This.

 

I recently changed my pads and my fluid was super low. After I changed the pads it was overflowing and I had to siphon it out. That should give you an idea of pads vs fluid level

 

Thanks for the quick replies everyone!

If in the cab, would it be up (coming down) the firewall? Or maybe down the brake pedal itself?

Pads were replaced with the new calipers about 10k miles ago. Rears were done about 20K mi ago. I'll look in the cab. I also just took a light and looked at the ABS module and nothing. I did see however on the booster it looked as if fluid had been or is there. With as slow as the leak is, I wouldn't expect to see anything appearing too wet. I wiped it clean and will monitor that. Maybe the gasket between the MS and the booster?

 

Could be, can't hurt to replace it and see. Gaskets aren't expensive so it's not like throwing parts at something, and with how old the rubber and gaskets are on these older trucks it can't hurt to replace them with new.

 

the brake fluid is being down past a weepy seal (NOT the one in the picture) at the brake master cylinder and is either sitting at the bottom of the brake booster rotting it out from the inside or being sucked into the motor.

When you replace your brake master cylinder I advise you to be careful when pulling it off and putting it on the brake booster. The seal in the picture is the master cylinder to booster vacuum seal and as you can see, it is damaged and this was the cause of a very intermittent P0171 lean condition, slightly rough idle... in other words, a real bitch to track down.

 

Good info to know. I’ve never done a master cylinder before just a slave on a clutch. Easy enough though. Booster I’m not familiar with...

After watching a few videos, that seems to be my thoughts. She’s probably just done. I’ll order parts and replace the old bits with new ones. Cylinder is pricy, but not seals and gaskets.

Is there any need to replace the booster you think?

Edit: Holy shit, booster is $721...

 

My son's dd truck was "losing" fluid recently. Refilled and within a couple days was low again. Had him take it to a reputable shop I trust. They diagnosed it as the booster and recommended replacing the master cylinder as well because of mileage/age and the fact that they'd be in there. They used parts from O'Reilly and it was $925 out the door. Only been about a month so I have no insight about how good the parts are yet. If you're interested I can get the p/n.

 

If it's still braking fine, I'd not change that.

The master and seals, yes.

If the leak stops, you know you had the right bit. Since you were losing such a small amount (comparatively) lets hope it was being sucked into the motor and just didn't create a running issue because it was so small.

You might inspect the vacuum check valve and hose to see if they are damp inside. That would be a clue.

 

I'm ordering a MC this morning.

There is a video on youtube that shows a guy with an F150 that had 28oz of fluid in his booster. He removed it with a pneumatic fluid evacuator. I'm hoping thats not the case for me, but I'll be picking up the extractor for $30 to be sure there is no fluid chillin in the booster....

The only real way to get it out is to remove it and I don't want to do that if I don't have to...

I really hope I don't have a lot of fluid in there...

 

If brake fluid were leaking into the cab you would smell it, especially at the rate that you mentioned.

 

The booster isnt particularly difficult to remove, just awkward as all the bolts are up on the firewall by the brake pedal assembly.

 

Why don't you do some DIAGNOSIS before you chuck parts at it?!?!?! It fucking boggles my mind. Remove the check valve vacuum fitting. Now you have a big hole in the booster. Take a zip tie, or similar bendy, non-breakable stick-like thing and fish it through the hole to the bottom of the booster. Just like you're checking your oil. It will either come out dry, or covered in brake fluid.


ALSO! The 'max' line on the reservior is the proper level of the fluid with BRAND NEW SHOES AND PADS AT ALL 4 CORNERS. The 'min' line is the *proper* level of fluid with WORN OUT SHOES AND PADS AT ALL 4 CORNERS. Fucking engineers thought about this shit, I promise you. IF everything is *really* right, your low fluid warning light will start to flicker just before you hit the backing plates.

 

I normally don't bite but...

Paetersen, I know all of that. Pads wear and cylinders fill with more fluid to compensate for the missing material.

What do you think I'm doing? It's called deductive reasoning, Watson. I've been working on cars for 28 years.

1: I've done new calipers pads shoes rear cylinders and drums in the last year or so and have checked pads and bled the lines in the last few months. Absolutely minimal wear.
2: I've known about the slow leak for a while now and have been checking under the truck periodically for signs of leakage to no avail.
3: Warning light has come on twice in a few months with no signs of leakage anywhere under the truck so it must be under the hood. I've been monitoring that for a few months now, but it wasn't a huge concern till the second light and really started to dig yesterday.
4: I saw signs of very slight fluid seepage under the MC yesterday and begun further research (Diagnosis if you will) into the MC/Booster.
5: Using my experience under the truck for the last 15 years as well as the experience of the nice people here who love to share their knowledge and deduced to reasonable (to me) satisfaction, that this is in fact my issue.

While your advice is absolutely useful, your prejudice and attitude isn't. I have plenty of money at my disposal to care for my maintenance needs, hardly your problem but I appreciate the concern. But I didn't acquire said money by throwing it away senselessly either. That's why I work on my vehicle.


FWIW, I had the vacuum tube off to check for fluid this morning and saw some signs of moisture in there. I did use your advice, and pulled it again along with the valve. Your help did in fact confirm my deductions, in addition to giving me an indication of just how much fluid was in there. It's a fair amount so for that I thank you for the tech tip. I'll be removing the booster as well and draining it properly.

In the future, just give the advice and leave the condescension and financial concerns out of it. It isn't productive. It's clear you have a bit of knowledge as well, and thanks for sharing.

 

You're either a massive dick in real life, or you had a really shitty start to your day today...either way you should learn to relax.

 

If we were face-to-face we'd be sharing beers, but the interwebs doesn't translate nuance and context very well, and this morning I'm particularly frustrated by customers giving me their 'interwebs DX'. The frustration wasn't directed at you personally, but directed at the trend for online forums to skip the DX part and just move straight to the chuck-parts-at-it part. The second paragraph wasn't directed at you at all, but seemed like information that is lacking in the DIY community, and most jiffy-lube type places too. I swear like a fucking sailor. I'm short a lot of the time. Take it or leave it, but there was no disrespect meant.

 

I'm an asshole. But I'm the asshole that will drop what I'm doing to get you off the side of the road at 2am with snacks. Not one of your nice friends that won't come help because it's too late.

 

I hang out with plenty of assholes but just try to remember some of the people on here are learning to DIY and we all start somewhere. The guy wants to work on his truck, that's better than 90% of people out there these days that would rather get their engine replaced under an extended warranty than be bothered to get the oil changed at a regular interval.

True story, I work with a 35 year old guy that had to get his tC's engine rebuilt for $5k because he thought the oil lantern light on his dash was a suggestion