Originally Posted by fastnoypi

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

Originally Posted by 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.

Originally Posted by 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.

Originally Posted by tus

Were you on a constant descent and using the brakes to hold 20mph?

If so you may have over heated the brakes, boiling the fluid in the calipers. The gas condensed back into a fluid when the system cooled and thing went back to normal (although you probably want to have the system bleed next time you have the brakes done.) Another thing that is fairly common when this happens is the rotors get warped.

Next time down shift the thing and use the engine to slow the descent.

Originally Posted by salimshah

I am still stuck at "pedal going to the floor"

Now if it was pedal depressed [not to the floor] but still no braking action then there are host of possibilities.

Graned in a panic situation it is hard to tell if the pedal is actually touching the floor.

Salim

Originally Posted by 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.

Originally Posted by KingGabe

**UPDATE**

Took the car into Lexus and they were unable to reproduce the problem.
There was however a history code that led them to determine the issue could've been caused by a faulty Brake actuator.
They will replace the actuator F.O.C. and I hope that is indeed the fix.

MellonC00, I know you mentioned this earlier but I don't think I had the same warning lights, although it's possible that they don't all turn on the same?

Thanks all!

Originally Posted by KingGabe

**UPDATE**

MellonC00, I know you mentioned this earlier but I don't think I had the same warning lights, although it's possible that they don't all turn on the same?

Thanks all!