I had to add brake fluid twice in about a month to maybe six weeks time, after some mountain driving. The light would come on; I'd add fluid and drive a couple weeks, then light pops on again. I crawled around underneath, but couldn't see a leak anywhere. I also felt in back of the master, but nothing.

So I took it to the local dealer (the one in Scottsdale is renowned for great, honest service) and they said it was due to the rear pads being so far down, and that the fluid was going into the rear pistons. They said they found no leaks including at the master cylinder. They said the pads were down to needing replacement, so I had them do it and turn down the rear rotors (first time).

So I'm wondering about two things:
1. Why would I have to add fluid twice in about a month? I've certainly experienced where pads wearing causes the pistons to use more fluid, but why would it be down twice in a few weeks, to the point that the light came on both times?

2. I had the rear pads changed at 198k and now have just under 235k, so why would the pads need changing again? I was in the mountains when I had them changed last time (198k) - do you think that mountain shop perhaps used really cheap pads . . . or didn't turn the rotors?

I had always had rears last 2-3 times what the fronts would before needing changed. But the service writer told me the the electronic proportional braking system may actually have the rears engage a couple nano-seconds before the fronts, to keep nose dive at a minimum. And supposedly, because I tow a trailer occasionally, the system might apply more power to the rears. But to have them needing replacement after only 37k doesn't make much sense to me . . .

I'm puzzled!

What do y-all make of this?

Interesting info. The fluid is entering and leaving the brake cylinders every time you touch the brakes. Having to refill in such a small time frame is odd to me unless there's a leak. If this was a different vehicle I would wonder if the brake booster is leaking fluid into the intake but these are run by motor not vacuum so not that. Cheap brake pads that wear quickly could account for it but they would need to wear extremely fast.

It does seem like these cars use the rear brakes just as much as the fronts which is unlike other cars I have had in the past. When I had a sticking caliper I monitored all of them with a laser thermometer and found the rear brakes seem to run as hot or hotter than the fronts. May be worth monitoring temps for a bit. Sticking (open) front calipers may force more braking to the rear. Sticking (closed) rear calipers may wear out the rears quicker. All things you can figure out by comparing rotor temps after braking or with little to no braking as in roll to a stop.

Yes, it is interesting and thanks for the info about the vacuum line not being a leak potential!

I just had the 4R back in so they could rework something (not brakes) they hadn't done right, and I had them check for leaks again. They doubled down on the leak, saying there wasn't one. But it is for sure an oddity (and also the pads wearing so quickly in the back). I will definitely be keeping a closer eye on those rear brakes!

BTW - someone else said they wondered why the rear disks weren't as large as the front ones on vehicles now, since they are being utilized more these days. It just occurred to me that even though the rears are being used more than they used to be, the front end gets more weight due to inertia (and also with the extra weight of a V8 on mine). Is that thinking correct?

Very interesting.

The rear pads should last 100k miles. If they arent, most likely your calipers are sticking or corroded and need replacement.

The front brakes still operate on vacuum from the engine. Only the rears are run by a mechanical pump. It could be a leak from the master into the vacuum drum on the firewall. Id start with the rear calipers, check for uneven wear on all 4 pads. If they are original, change the calipers out. If the fluid still seems to vanish, look at the master cyl. That would be a service shop job.

Ah, I never realized this.

I don't believe that is correct, I don't have a "vacuum drum" on my 2004 That I have ever seen, or a vacuum line either, I will look closer when I get the time hopefully tomorrow or the next day.

Also a "vacuum drum" is called a vacuum booster and it also doesn't show up when looking at online dealer parts website in the diagram of the brake system.

Yeah, I don't see any drum either, but maybe they're using vacuum in another way perhaps?

I just looked at my truck and see no vacuum lines going to the brake area and certainly no bulbous booster. I figured I missed something but I guess not this time.

You are correct. My mistake.