Brake fluid boiling, why?

[JHMCB7[Linus]]

Ok, so i was driving home after work, its like a 10min drive, i got home. After about 45 mins i get back in the car and drive about 200 yards, i break to slow down and feel the brake pedal kicking like when the abs kicks in. And at the same time i hear a fizzing noise coming from the engine bay. I stopped by the side of the road, popped the hood and see the brake fluid boiling over the container. How could this happen? I have heard of brake fluid boiling on track days, but never during a normal drive.

I have done somer reserch and the only reason i see is the breakfluid is old and probably has water in it. But could there be any more serious reason? Are there any cooling of the fluid that could be malfunctioning? Btw i just fixed my abs wich has not been working for over a year.

[visualpoet]

You could have a caliper sticking slightly, that would overheat the fluid.It's also possible it needs replaced.

[JHMCB7[Linus]]

You could have a caliper sticking slightly, that would overheat the fluid.It's also possible it needs replaced.

Would i be able to feel that if i raise up the car and rotate the wheels? Also, if the abs system has not been working for about a year could it be that the abs kicking in it puts more force on the fluid then when i brake normally?

[visualpoet]

You should be able to feel it, yeah. I can't see the ABS causing it. It generally cuts pressure to the brakes, unless something is really wrong with it.

[deevergote]

The ABS system has it's own reservoir as well, doesn't it?

I agree with the sticking caliper. If you're putting more friction to the pad than normal, you're creating plenty of heat just by driving. It's not the braking that boiled the fluid, but the driving itself.

Also make sure that your pads aren't worn down to the metal. The pad material can absorb heat. If it's bare metal, it'll transfer it straight to the fluid. I had an inner pad that was down to the metal myself... by looking at the outer pad, you'd never think the inner would be so worn!


Also, how hard are you braking that you're making the ABS kick in? ABS should only kick in when you slide on wet or icy roads, or in a panic stop. If your normal driving habits activate the ABS, you might be driving/braking quite hard.

[92bently]

yeh i have seen brake fluid boil just from daily driving, if its never been changed it has asorbed a lot of moisture over the years and the boiling point is proly very low

[deevergote]

That's very true... old fluid is far less effective. (my clutch fluid was brown! )

[JHMCB7[Linus]]

well i just tightened my handbrake, so there might be more friction on the pads then normal. If the pads were worn to the metal, wouldnt it make a grinding noise when braking? But still i thing it was just bad fluid, i just changed it and went for a testdrive, and the only problem i got was the police telling me to turn my foglights off haha

[JHMCB7[Linus]]

Also, how hard are you braking that you're making the ABS kick in? ABS should only kick in when you slide on wet or icy roads, or in a panic stop. If your normal driving habits activate the ABS, you might be driving/braking quite hard.

My abs is only kicking in when i slide, it works normal i think. What i ment was when the fluid started boiling the brake padal was kicking, almost like when the abs activates even though i only braked slightly to slowdown. When i normally drive the abs doesnt activate, so nah i dont drive so hard, only occasionally

[deevergote]

Ah, ok. I thought you said that your ABS activated and then it started to boil. I didn't realize that you were just using the ABS feeling as a reference to what the pedal was doing. Gotcha

[Night Wolf]

water/moister in the brake fluid can lower the boiling point of the brake fluid. thanks to a chemical in the brake fluid the abosorbs moister

[johnl]

There's not much that's more serious than a significant amount of water in the brake fluid. This lowers the fluid boiling point, hot fluid boils (suddenly) generating gas in the hydraulics, pedal goes to the floor (because gas is compressible as opposed to liquid which isn't), and the car won't stop, with obvious implications.

Brake fluid is mostly ethylene glycol (i.e. a type of alcohol) which is hygroscopic (attracts / absorbs water molecules). This happens gradually over time because to allow pressure equalisation the MC reseviour isn't entirely sealed from outside air, which has moisture content (to state the obvious). Water in the BF also causes internal corrosion in the braking system. It's a good idea to change BF annually (which reminds me...).

If this only happened just after you adjusted the handbrake then it's not unlikely that you overadjusted it and the rear pads are dragging, creating a lot more heat in the rear brakes...

[foamypirate]

water/moister in the brake fluid can lower the boiling point of the brake fluid. thanks to a chemical in the brake fluid the abosorbs moister

Brake fluid is alcohol based, and absorbs water. This is how fuel treatment works; since they are alcohol based, when put into the gas tank, they will absorb the water and allow it to mix with the gasoline, diluting it to the point that it doesn't matter anymore...

[Karl]

I have experienced pad and fluid fade to the point the car didnt want to stop, but only on the track, and even then the fluid in the resivoir never boiled.

[tear2wo]

try bleeding the whole entire brake system. then see if it still boils. if it still does it could be a sticking caliper.