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So I'm having a bit of trouble with boiling fluid and I'm not really sure what I can try at this point.


The first occurrence was at the AROCA Phillip Island day 3 weeks ago. I had been running Penrite SIN (Now Penrite Racing) fluid - a 600F fluid. It was only fully flushed a year ago, but had been bled before every track day (3 Winton days before the PI day). After my second session, pedal was squishy and my braking ability had significantly decreased. So I bled all 4 corners again, seeing big bubbles and blackened fluid. Put down a few more laps and had the same issue.

Assuming that the issue was due to fluid absorbing moisture over time, I did a full brake fluid change with TRW GP600 (supposedly an upgrade over Penrite Racing). Used about 750mL to get all the old fluid out and even manually forced a few ABS cycles using the method described here. I also removed the dust cover plates from behind each rotor for extra cooling.

Then last week I went to Winton for the WMR Sprints. After my second session, and only ever doing 2 hot laps consecutively, sure enough, pedal went soft again. Did a brake bleed and got HUGE bubbles coming out my front left and rear right calipers - the other two were fine.

A little lightweight MX5 shouldn't be able to overheat GP600 fluid, especially not when I only did a couple of consecutive hot laps. Something else is going on.

Anyone have any ideas, or have run into similar issues?

Do you have speed-bleeders?

I seem to recall somewhere on the forum in the past that people were having issues with them allowing air to get back into the lines.

Nah, just been doing it the old fashion way. Put silicon tube on bleed valve, press brake 50% of the way and hold, open bleed valve and let some fluid come out, close bleed valve, let go of pedal.

What's your pad wear like? If the whole system is running super hot due to a bad slider or corroded brake piston not releasing fully you may just have the fluid as the weakest link in your system but I'd still expect some massive taper or plain big wear on the pads. With my last trackday for instance I must've been over 1000C to destroy the XP12's pads in one day (they're race pads and never faded but wore super fast)... my pads got hot enough that the backing plate distorted between the wilwood's pistons but no issues with my RBF600 fluid. The pedal was solid all day and I don't think RBF600 is that much better than either of those fluids. If the whole system isn't getting stupid hot..then you're getting water/air in the system somewhere.

Jarik wrote:Nah, just been doing it the old fashion way. Put silicon tube on bleed valve, press brake 50% of the way and hold, open bleed valve and let some fluid come out, close bleed valve, let go of pedal.

That's a little vague and I don't see why you quote %50 of the pedal. Personally I always bug a friend and do.

1) Person 1 opens Bleed Valve
2) Person 2 Pushes brake pedal until they hit the floor.
3) Person 1 closes Bleed Valve
4)Person 2 then releases the brake pedal
5)Repeat until person 1 observes no air bubbles out of the bleed point

Order is furthest from the ABS unit to closest. I've never had to cycle the ABS unit to get a solid pedal personally

For the record (sorry off topic), I've been using speed bleeders with success. I ensure quick short pumps so the check value is moving quite quickly, to ensure a good seal. I digress. I doubt there's anything wrong with your ability to bleed.

Could try the gravity bleed method for something different to try. As always, doing the same thing expecting different results won't help. So I agree with Hellmun, something, somewhere is allowing air into the lines, because personally haven't had the issue of boiling fluid before. Good luck diagnosing the problem.

Hellmun wrote:What's your pad wear like? If the whole system is running super hot due to a bad slider or corroded brake piston not releasing fully you may just have the fluid as the weakest link in your system but I'd still expect some massive taper or plain big wear on the pads. With my last trackday for instance I must've been over 1000C to destroy the XP12's pads in one day (they're race pads and never faded but wore super fast)... my pads got hot enough that the backing plate distorted between the wilwood's pistons but no issues with my RBF600 fluid. The pedal was solid all day and I don't think RBF600 is that much better than either of those fluids. If the whole system isn't getting stupid hot..then you're getting water/air in the system somewhere.

Jebus....My XP12's will see me through two seasons of motorsport! Something is going very wrong there.

You can get bubbles in the system from failing seals in/on the master cylinder.

First cause: Failing seal on the master cylinder piston. Press pedal down - some fluid escapes into the driver's footwell. Lift foot off pedal - some air is drawn into the cylinder. Any sign of fluid leaks around the brake pushrod in the footwell? Replace master cylinder.

Second cause: leaking seal between the master cylinder and the fluid reservoir. Any sign of leaks on top of the master cylinder? Replace master cylinder or just that seal - your choice.

If both look good then check for fluid leaks on all lines and fittings on the entire brake system.

deviant wrote:Jebus....My XP12's will see me through two seasons of motorsport! Something is going very wrong there.

Yeah, the speed Mark gets and the late braking is way higher/later than us mere mortals.
My XP12's are 50% worn after 8 track days.

I'm with manga_blue, seals are gone letting air in. You say the fluid is "blackened". This could be broken down black rubber seals coming out with the fluid.

Double post

deviant wrote:Jebus....My XP12's will see me through two seasons of motorsport! Something is going very wrong there.

They just overheated, bits of XP12 pad were all over the rotors...and I've never bent a backing plate before! When I had 90kw though I'd still only get 8-9 trackdays out of them on the sport brakes by the end (wear got worse as I got better times). The cars about 190kw on low boost now so until I do more testing and get temps back below 500C consistently I'm going to keep getting shocking wear. I'm still impressed I didn't notice... no horrible brake smells or loss of pedal even though I was over 1000C. The rotors would've been glowing...

Sorry about the delayed reply, was a busy week doing my first clutch change. =)

Anyways, thanks for all your suggestions.

What's your pad wear like? If the whole system is running super hot due to a bad slider or corroded brake piston not releasing fully you may just have the fluid as the weakest link in your system but I'd still expect some massive taper or plain big wear on the pads.

Inner/outer pad wear seems roughly even across each caliper. I haven't had a chance yet to pull out the pad and check for taper, but just looking through the back of the calipers seem roughly even.

That's a little vague and I don't see why you quote %50 of the pedal.

I could be wrong, but I've heard you should avoid to let the pedal go further than what you'd use in normal braking since it can rip the master cylinder seal if gunk has built up there.

You can get bubbles in the system from failing seals in/on the master cylinder.

First cause: Failing seal on the master cylinder piston. Press pedal down - some fluid escapes into the driver's footwell. Lift foot off pedal - some air is drawn into the cylinder. Any sign of fluid leaks around the brake pushrod in the footwell? Replace master cylinder.

Second cause: leaking seal between the master cylinder and the fluid reservoir. Any sign of leaks on top of the master cylinder? Replace master cylinder or just that seal - your choice.

If both look good then check for fluid leaks on all lines and fittings on the entire brake system.

No signs of leaking either in the engine bay or in driver footwell. Is there any other way of checking the seal between MC and reservoir? I should note that with a fresh fluid change, the pedal has a nice hard feel (I've had a failed MC piston seal on another car and the pedal felt squishier).

Don't think it's the master cylinder, as the bubbles are appearing at the caliper end of the lines. Don't bubbles tend to travel UP the system?

Could try the gravity bleed method for something different to try. As always, doing the same thing expecting different results won't help. So I agree with Hellmun, something, somewhere is allowing air into the lines, because personally haven't had the issue of boiling fluid before. Good luck diagnosing the problem.

Might try gravity bleed with my next fluid change.

The only thing I can think of is that potentially the old fluid had such a high water content that it mixed in with the new fluid. Seems kinda bizarre though.