9KCanuck

Hopefully someone can explain what happened to me... I have a theory, but it's the first I've experienced this, so just educated guessing on my part.

Yesterday I swapped out my half-done Stoptech Sport pads at the front for new ones, just to be absolutely sure I had enough pad for the track today. I obviously had to push the pistons in a bit due to new pads being thicker. This is all 1st grade stuff so far.

BUT, because I often go too quickly and don't THINK (especially when I'm kinda panicking to get the car ready and all my stuff for the track the next day), I didn't suck out any fluid from the MC reservoir. It was WAY over max line, which I found out at the track.

Anyway, on the street yesterday and driving to the track today, the brakes were 100% fine. No issues whatsoever. And first lap or 2 of the track, no problems. 3rd lap I got a mushy pedal and fade. Weird.. so I take the next lap a bit slower to see if I'm imagining things, but no... brakes definitely not good at all.

In the paddock, I see that my left front is smoking. Not a lot, but definitely smelly tendrils coming off the caliper. And the fluid reservoir had apparently spat out some fluid into the engine bay. *Unless* this happened when I pushed the caliper pistons back in the day before (because it was that overfull, I think).

I let the brakes cool, and I sucked out enough fluid to get it below the MAX line, then left the track for a quick drive on the highway and did some moderately hard stops (as traffic allowed) -- brakes totally fine. Lots of bite, no fade. Awesome!!

So next session, you know what's coming -- after only 2 laps, the brakes start to fade again. Not as bad as before, but also this time I was ready for it and got off the track right away, so they probably didn't get a chance to heat up as much as first session.

My theory (please tell me if this is possible):
- due to a WAY overfull reservoir, as the fluid heated up and expanded (presumably even brake fluid will do this), it had nowhere to go other than out the top and/or pushing the brake caliper pistons out. That left front caliper got 'stuck' engaged due to this pressure, and then feedback loop -- hot caliper, fluid expands, nowhere to go, pushes caliper, etc.

My *questions*:
- Basically is my theory correct? Can an overfull reservoir really lead to that? Will it really not just relieve ALL the excess pressure through the cap? Or is that reservoir and cab robust enough that the pressure could relieve itself partly by simply pushing the pistons out?
- Did the brake fluid BOIL and lose its heat tolerance? Is that why it was OK for only a couple laps the second time out?


Car is AP1, stock calipers all around.
Motul RBF 600 fluid + stoptech sport pads + stoptech slotted rotors all around. These pads/rotors/fluid have always been more than adequate for me at this track.

B serious

No, an overfull reservoir will not cause a mushy pedal.

How old is your fluid? If its over 1yr or a few track events, it may just be full of water and need to be flushed.

Or your caliper is stuck.

Or your pads are overheating, not heat sinking properly, and putting too much heat back into the caliper or causing knockback or excess movement or flex.

9KCanuck

Thanks for the reply.

The fluid has seen maybe 2 events. It's less than a year old, but perhaps not as fresh as I think it is. And perhaps getting it so hot today ruined it, if that's a thing. I'll bleed the whole system and put new stuff in for sure.

I feel like the new pads did get glazed or whatever, so you're probably right. I never did feel like a caliper got stuck, though. Presumably I would have felt a tug to the left in that case? I think part of the problem is that I have used these same rotors for both track and OEM pads. They have had track pads in for a few weeks now, though... so putting in new pads of the same brand/model should have been fine. But I didn't bed them or anything -- just put them in last night and track today. Maybe that was not smart.

I gotta bite the bullet and just get dedicated track rotors (no slots, no holes) and keep the pretty slotted ones for the street. Brakes are such a headache.

B serious

Stoptech Sports are on the edge of what works as a track pad. Maybe you're just out running them

9KCanuck

I wish that were the case, but I'm not that fast. These pads have been completely fine for at least 2 track days in the past. For my level and the track I go to, they're more than adequate.

I think something about my procedure is to blame, I'm just not sure what I did wrong. But it almost is like the pads I put on (from an unsealed box and plastic-wrapped) were the wrong pads. Unlikely, but it would kinda make sense. Maybe when I get the calipers off, I'll see something interesting. The over-filled reservoir was obviously a red herring.

I have one more track day at the end of the month, so I'll need to sort it out.

B serious

A soft pedal means air bubbles in the fluid. The fluid boils and the pedal isn't capable of creating pressure.

If you really did have boiling fluid, maybe you left some old fluid in the system when you did your last bleed. It takes at least about 1 liter of brake fluid to fully flush out the S2000's system. A 1 man system works best. A 2 man system is arguably the least efficient.

However, when pads fade, the pedal feels stiff, but no friction is being created. Its described as feeling "wooden". It can be mistaken for a "soft pedal" because this often makes people panic and push a ton more pressure into the pedal...which, of course will cause the pedal to sink.

Pad fade can also lead to fluid boiling.

Maybe that helps with your diagnosis.

You can confirm your pads are stoptech sport by reading the part number printed on the top or bottom face of the friction material. The part number should start with 309

9KCanuck

Yup, that makes sense and it helps. I'll check of course, but the symptoms definitely point to the fluid getting toasted. Perhaps just because it was old/used enough, OR whatever caused that caliper to get so hot it was smoking did the fluid in. That caliper bit is the mystery to me. Who knows, maybe I put a pad in backwards or some other install goof.

thanks again.

9KCanuck

New theory, but this one still involves me being a dope. Just a different mistake.

I just remembered -- when I was swapping old pads for new, I had no internet so couldn't check the torque for the caliper bolts. I was pretty sure it was 40ft/lb though, so that's what I torqued them to. I googled it just now, and the correct value is 24.

Could this over-torque have decreased clearance between new pads and rotor *just* enough such that once they got hot, they expanded (I assume even brake pads will expand even a hair when very hot?) and there was sustained pad+rotor contact for some period of time even while not braking? That heated up the pads enough to smoke and perhaps boiled the fluid, or at least rendered it no longer adequate for track purpose.

Anyway, regardless of what, if anything, expanded or whatever... my theory is that the root cause of the issue is over-torqued caliper bolts.

Think about this fact, though: somehow I have survived 50 years. That's remarkable.

B serious

No, overtorqued bolts would not squeeze the pads into your rotors or make the caliper harder to slide. All they do is secure the caliper to the bracket.

I would, however, buy new bolts now. Assuming you're referring to the 12mm headed caliper bolts. 40LB-FT most likely has damaged the bolts.

Not related to your issue.

9KCanuck

Good call. I'll replace the 4 bolts and maybe the pins as well just for fun.

Speaking of the pins, I'll have to check when I take them apart again that I've got each in the right spot. It's possible I have the flat one in the wrong spot on the one that overheated.