losing brake fluid
 

My kid's 99 4 runner is losing a bit brake fluid. It goes below the low mark in the reservoir within about a year's time. It gets driven only one week out of the month. I can't seem to find the leak. I looked around the wheel for leaks, nothing. Dry around the master cylinder. Where else? I haven't open up anything yet.

Power brakes?

With power brakes, fluid can leak out of the back end of the master cylinder into the brake booster.

I crawled under there to see if there's any wetness. its bone dry. That was the first place I looked. Looks like I have to remove the wheels and the drums.

Unless you pulled the master cyl off of the booster, you probably wouldn't see anything. The fluid gets trapped inside the brake booster, it's essentially a big air tight reservoir.

One possibility is pad wear.
The pad thins, the pistons extend, then more fluid is taken from the reservoir.
Organics (vs metallic) wear faster afaik and I thought easier on the rotors. Could be wrong. BMW wheels are always dirty.

Other possibility might be inside the brake booster.

Yep, as the pads wear and the pistons extend, the fluid level gets lower because the system volume increases. I didn't mention that because it only gets driven one week a month, but I guess if that week is hard on the brakes...

DOes the pedal sink when you hold the brakes? Internal MC leak?

rjkp

No, you have to remove the MC. As others have hinted, the missing fluid will be inside of the booster, unless you have a leaky wheel cylinder in the rear drums, which should show up without removing anything from under the car.

If it’s not inside the brake booster, look on the firewall and check the carpet.

Got it.

Car has less then 10,000 miles on new rotors, pads and new brake lines. I am thinking its the master cylinder too. I am so tempted to just rip it out and replace. It has over 220,000 miles on it.

If/when you remove the master cyl, make sure you get the brake fluid out of the brake booster (assuming it's in there). I've never removed a brake booster. I'm not sure how much of a PITA that is. Maybe you could pump the stuff out.

Replace the booster too. Instant vacuum leak when it fails.

Pedal holds pressure. Just tried it. Now I am thinking its the wheel cylinder?

You can ask questions but ultimately you need to inspect the car and diagnose it. You can't inspect a car through an online chat.

I know, just trying to see if there's anything else I am missing. Will make him pull the master cylinder when I get the new one. Its time to get rid of it due to the mileage even if its good, its time after 200,000 miles. Aftermarket from Places like R0ckauto or dealer?

Get it from BMA in Glendale. They sell Japanese OEM parts for good prices.

Thanks Dennis. Will call them tomorrow. That place is 15 min from my house. Very cool and no traffic