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Just having flushed my brakes I checked the master reservoir while the car was hot. I drove it about 25 miles up an interstate then finished with 3 miles of surface roads. No hard braking, just a couple easy stops at traffic lights. Ambient temp was 85F. Anyway, when I went to remove the cap I damn near burnt my fingers. I used an IR thermometer to measure the temp... see photo. I also checked the rotors and calipers and all were in the 112-115F range... IOW, cool. So what is putting the heat into the brake fluid? I know this is way below what it can take. Just seems strange and I wonder if maybe I have some brake lines that aren't properly shielded from the exhaust or whatever. Anyone else seen this?
I'll bet your windshield washer fluid is hot too. There's a lot of heat under the hood. Did you check temp of any part of the engine, radiator, or alternator?
Yep, sounds normal to me, too. They get heat soaked, and 115 degrees F sounds about just right. Just idle your car in traffic, and you'll see air intake temp also jump to that exact range (if you can monitor it).
yea the reservoir is right over the exhaust manifold, and that area does not have prime airflow. This is the reason why you don't want to overfill the reservoir. If the fluid heats up and expands enough, you could start putting on the brakes (so to speak).
?? Yes, the rotors were ~115, that is indeed normal and cool. The brake fluid was 180+. Maybe normal for a C7... I dunno, but I have not seen anything like it before.
That 180+ Still seems like a lot of heat for just being in the engine compartment. That is about equivalent of coolant temp and this is nowheres near the radiator. I run a scan gauge 2 on my Tacoma and one of the parameters I have up on the screen is IAT and when the truck is up to temp it runs only about 10-20 degrees above ambient. But that is a much more airy engine compartment. Wondering if it might be just because the tight C7 engine compartment has the reservoir so proximal to the drivers side exhaust manifold.
Anyway, just curious... burning my fingers on a master cylinder reservoir is a new experience for me. And if you saw the photo I posted of my fluid pre-flushing it was pretty brown. Maybe partly a result of this constant slow cooking. If so, frequent flushes are definitely in order. I'll check the MC temp on my truck next time I drive it and maybe a few other vehicles and post up the results.
Originally Posted by X25
Yep, sounds normal to me, too. They get heat soaked, and 115 degrees F sounds about just right. Just idle your car in traffic, and you'll see air intake temp also jump to that exact range (if you can monitor it).
IAT has nothing to do with underhood temps. That is the temp of the air in the intake which on a modern car is ducted to outside of the engine bay.
This makes perfect sense that the brake fluid is that hot in the reservoir as it sits 6" from the headers which at cruise are around 1000*F. The temp of the brake fluid that matters is out in the calipers. I bet that was much closer to rotor temp.
It has a little to do with it because the filter, housing, and ductwork transfer some heat to the intake air. See my Tacoma whose IAT usually runs 10 to 20 above ambient.
Yes, I agree that it is probably the exhaust header causing the brake fluid reservoir to heat up. Not an issue with my Tacoma since it has a lot of separation.
Originally Posted by subieworx
IAT has nothing to do with underhood temps. That is the temp of the air in the intake which on a modern car is ducted to outside of the engine bay.
This makes perfect sense that the brake fluid is that hot in the reservoir as it sits 6" from the headers which at cruise are around 1000*F. The temp of the brake fluid that matters is out in the calipers. I bet that was much closer to rotor temp.
You should take the car to the track and do a hard half hour session. Everything gets really hot then. It is easy to burn your fingers on almost anything you would grab under the hood.
Hot brake fluid... why?
