Are self bleeding brake systems a thing?

"TheTurbochargedSquirrel" (thatsquirrel) 09/03/2019 at 17:45 • Filed to: None

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So I trackcrossed the Corolla on Sunday and the torque vectoring system did a nice job of heating up one corner at a time to fluid boiling temps. When I left the pedal was mushier than it was before the event, not bad, but not as firm as it was. Fast forward to today and its back to almost exactly how it was when I bought the car 1000 miles ago. The sponginess is completely gone and the pedal is back to brand new levels of firmness. Only evidence of the fact that I boiled the fluid is that there’s now about .25" more travel before you get to where the pedal firms up than there was before.

So what kind of witchcraft is going on here? There was air in the lines and the pedal was spongy when I left the event. Then you let it sit over night and the pedal magically fixes itself. I boiled the fluid in the Mini on multiple occasions (Hawk HP+ are way too hot of a brake pad for that car) and every time the spongy pedal stuck around unchanged until you spent all day cursing as you desperately  fought it to get a proper bleed out of the brakes. This one magically fixes itself overnight. Are brake systems that can evacuate air from the lines on their own a thing? Why did my pedal feel come back after letting the car sit?

jimz > TheTurbochargedSquirrel 09/03/2019 at 18:04

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there wasn’t air in the lines, it was bubbles of vaporized brake fluid.  When it cooled off, the vapor condensed back into liquid.  “Boiling fluid” fade doesn’t introduce air into the system.

VajazzleMcDildertits - read carefully, respond politely > jimz 09/03/2019 at 18:18

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Y eah, as I understood it, it’s a closed system due to the hydraulics involved. So it would just be existing fluid changing physical state and returning once energy levels are back to normal. 

jimz > TheTurbochargedSquirrel 09/03/2019 at 18:18

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as for there being more initial travel before firming up, I wonder if your track session caused some runout in the rotor(s) which are knocking the pads back a bit more as they turn. 

SilentButNotReallyDeadly...killed by G/O Media > TheTurbochargedSquirrel 09/03/2019 at 18:27

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It's not magic... it's physics.

AlfaCorse > TheTurbochargedSquirrel 09/03/2019 at 18:33

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Are you sure you boiled the fluid? That should be extremely hard to do, especially so on brand new fluid. If the pedal feel comes back, even if it’s not 100% what it was, it is almost certainly the pads. If the brake fluid truly boiled, the pedal would never firm up on its own even if completely cooled down. More track oriented pads I’m sure would help out a lot.

TheTurbochargedSquirrel > AlfaCorse 09/03/2019 at 18:51

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I hon estly expect that both the pads and fluids are a bit worse for wear after sunday’s activities. My gut says boiled fluid because it never lost it’s stopping capabilities, the pedal travel just got longer and softer. Pads are a real possibility though, they were getting smelly on whichever side the torque vectoring was dragging each run.

Once the stock stuff is worn out I’m going to do a full brake overhaul with it. Flush for higher temp fluid, drilled and slotted rotors, higher performance pads, and cooling ducts in the front. I’m not going to go as aggressive with the pads as I did on the Mini, the HP+ I put on that were a pain to live with. I’ll probably go for an aggressive street pad instead of a full on trackday pad.

TheTurbochargedSquirrel > jimz 09/03/2019 at 18:53

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I thought brake fluid broke down when boiled and didn’t return to it’s original state afterwards. At least that’s the reason I have been given by a lot of people for why you should bleed or flush your brakes after every track day. The Mini’s pedal certainly never came back after any of the times I boiled it’s fluid (Hawk HP+ are way too hot of a pad for a Mini with no cooling ducts in the front).

TheTurbochargedSquirrel > jimz 09/03/2019 at 18:54

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That’s what I was thinking was likely. I’m perfectly OK with a slightly longer travel as long as it still firms up and stops like it’s supposed to.

AlfaCorse > TheTurbochargedSquirrel 09/03/2019 at 19:23

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Sounds like a good plan. On stock pads, the pads are typically what you notice fading and getting smelly first,   long before the fluid boils unless the fluid is heavily contaminated. So I would start there, but it is definitely good to get higher temp fluid too.

BTW I learned on a trip   in Cuba that instead of replacing brake fluid after a few years , they will remove the fluid and boil it on purpose to boil off the water that had been absorbed in the fluid and then reuse it. Either that or they use an alcohol/ shampoo/ sugar mixture as a brake fluid. I’m not sure which is better haha

TheTurbochargedSquirrel > AlfaCorse 09/03/2019 at 19:26

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On one hand I’m super impressed with the stuff Cubans come up with to keep their cars going. On the other hand I’m quite glad there's an ocean between me and those cars.

jimz > TheTurbochargedSquirrel 09/03/2019 at 19:31

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when it breaks down it leaves crud/sediment which settles to the bottom of the caliper bore and can eventually cause it to stick.

Arch Duke Maxyenko, Shit Talk Extraordinaire > TheTurbochargedSquirrel 09/03/2019 at 20:18

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Regular ass street brake fluid deteriorates rapidly at high temperatures, just like any other hydraulic fluid. Picture it being like strands of cooked spaghetti that get cut up with a fork, the more cuts you take, the less resistance you have to pushing through the spaghetti.

Arch Duke Maxyenko, Shit Talk Extraordinaire > TheTurbochargedSquirrel 09/03/2019 at 20:19

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Don’t use drilled rotors for track day bro shenanigans. Slotted is fine, drilled will crack.

DipodomysDeserti > TheTurbochargedSquirrel 09/03/2019 at 21:10

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I once had a ‘66 GMC with self bleeding brakes. They’re pretty scary.