KingGabe

Hello all,
Just this passed weekend we were in the mountains in my wifes 2015 RX350 AWD I had the 4 wheel lock turned on and going under 20 mph. Within about 15 minutes or less the brake pedal went to the floor and I had to use the e-brake to come to a complete stop.
I checked the fluid level, and for leaks, all was fine. Let it sit for about 20 minutes and then the brakes were working fine again.???
Has anyone else had this happen? The instrument clusters Brake light did come on but that was when it was too late.

Htony

Over heating pads?

salimshah

Heat base braking loss happens due to the failure of pads to the disc friction, but the pedal going down to the floor happens due to issue with fluid compression.

I would look for leaks [drop n the level in reservoir]. If that did not happen then have the master cylinder checked.

Salim

fastnoypi

when was the last time the brake fluid had been flushed/replaced?

KingGabe

The car only has about 25k miles so the fluid has never been flushed. The reservoir was full and I didn't see any leaks.
It worked fine after sitting for about 20 minutes, that's the confusing part of it to me.
Does anything else engage when turning on the 4 wheel lock other than the obvious??

MellonC00

does your situation sound like this?
https://www.clublexus.com/forums/hyb...eed-issue.html

KingGabe

Not exactly. Her RX is not a hybrid, and the pedal went down but never kicked in. I even tried to pump the brakes to stop with no pressure build up.
I had to use the e-brake to come to a complete stop. It was a normal slow down to a stop situation not emergency braking fortunately.

MellonC00

How about this issue?
https://www.clublexus.com/forums/rx-...lfunction.html

KingGabe

I don't remember seeing the abs or traction control light, just the brake light.

KingGabe

By the way, Thank you all for your help and input into this problem so far.
I love these forums for the quick response and willingness to help from other Lexus owners!!

fastnoypi

IIRC, the manual recommends changing brake fluid every 3 years or 30k miles which ever comes first. You may have air or moisture in the system.

RxDuderino

ToledoRX

I was going to say that brake fluid is hygroscopic (absorbs moisture from the air) so if it's been a couple of years, there is probably moisture in the brake line that could have boiled and turned to steam during prolonged braking. If that is the case you may be able to see condensation in brake reservoir.

If your 2015 RX was made in 2014, your brake fluid is probably 4 years old (mileage doesn't matter as much as age). Maybe a brake flush can help - though I would chalk it up to experience and not drive for prolonged periods down mountain roads.

coolsaber

have you tried replicating your problem in the same situation in a safe and controlled area? Im not sure about the way center diff lock works, but awd drive systems tend to aggressively use braking as a form of control. Now someone posted fade vs fluid compression issues and that is something to look over. How does your vehicle handle mountain or constant high to low brake applications. If anything, get the code read from the brake light if anything, it should be a stored diag code.

N4TECguy

I am not 100% sure, but here's my thought. The RX likely uses 3 open differentials. Open differentials send power to the wheel with LEAST resistance, so if you have a spinning tire, that's the one that gets power. Obviously, this is useless. Lexus (and most CUVs in the industry) use the brakes to create resistance, thus sending power to more useful wheels, the ones that still have traction.

4WD Lock tries to send power to the rear wheels under 25mph. (I think...) there is no mechanism to accomplish this, other than to apply a bit of brakes to the front wheels. Since you were crawling under 25 the whole time, it just kept applying brakes since you kept it locked. This subsequently overheated the brake fluid due to constantly being applied. The brake fluid boiled, and air more than a liquid...hence your pedal going to the floor. The emergency/foot brake does not run off hydraulic pressure (your aformentioned brake fluid), but instead uses a cable mechanism, so it was unaffected by the rest of the system and effectively brought you to a stop. Within 20 minutes, the brake fluid cooled enough to operate as usual.

That's all I can think of. IF this scenario is true, any competent mechanic who inspected your brake fluid and bled the system (which you should DEFINITELY do now) would be able to tell if the fluid overheated.