Porsche Brembo 996 BBK Conversion-- let's figure out how to fix the dust boots

[Obioban]

Disappointing that there's no easy solution here. Ian - if I remember right you started this conversion to fit a BBK under 17s, did this end up working?

They fit under the wheels I was running (APEX ARC8s in 10" ET25, but I got annoyed with the front offset and small amount of tire options in 17"... so I'm on 18s now.

I'm not sure there's no easy solution here. Seems like some blue high temp dust boots and titanium heat shields, and we may be good to go.

[blazenXLT]

Disappointing that there's no easy solution here. Ian - if I remember right you started this conversion to fit a BBK under 17s, did this end up working?

[Obioban]

Maybe this is a stupid question - but do the stock M3 boots disintegrate or are they fine?

Disintegrate.

[Slideways]

I want to say that the last few 996/7s I saw at the track had aftermarket BBKs on them. It seems a little ridiculous to expect heavy track use out of a caliper designed for a street application.

[cobra]

Maybe this is a stupid question - but do the stock M3 boots disintegrate or are they fine?

[mrgizmo04]

Also, a lot of posts use seals and dust boots interchangeably, in some contexts incorrectly.

Pressure seals or seals or caliper seals normally refer to the ones that go inside the caliper bore and has the pistons slide against, holding the fluid from spilling out and are not visible unless the pistons are completely removed from the caliper. They seal the piston bore and the piston.

Dust boots or dust seals (sometimes just referred to as seals, incorrectly) are the boots that prevent contamination from getting into the piston/caliper bore and are the ones that melt first behind the pad backing plate.

Thread here is for the dust boots.

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[mrgizmo04]

Brakes. My favorite topic.

My experience with Stoptech seals/dustboots on 996 kit.

I wanted to rebuild the calipers when I got them used. After installing the Stoptech seals into the bores I went to press in the pistons. I needed to use A LOT of force with my fingers and in pretty much all bores needed to use the caliper spreader to push the pistons in (I could not simply do it by hand, not normal feeling compared to all the calipers I rebuilt in the past). They did feel tighter than what I expected as normal, but I had a track day coming up, so I rolled with it.

I run my track pads rather low and swap to street pads, so the pistons get a good workout with being in/out of the bore. After a few events on inspection I noticed a bit of fluid running down the caliper and a rip/hole in the dust boot (first pic).

I thought of just replacing the boot, but something told me to just rebuild the entire caliper again, given how tight the seals were when I installed them and the slight accumulation of fluid. Removing the dustboots uncovered shreds from the seals (second pic with black dust).

I went with Girodisc kits to rebuild. When pushing the pistons in by hand, they felt "right" in terms of resistance with their seals. There must be some small bore differences and seal tolerances between Stoptech (made for Stoptech caliper/piston/bore) and Girodisc, even though they are stamped the same bore. Now interesting thing with Girodisc dustboots, they come out of the bore...not rip, just come out, they are just slightly too small in the bores and when pistons come out they must get pulled out (third pic on bottom right). I found this on several of the pistons. Before this turns into a "Stoptech sucks" thread, Stoptech seals and dustboots actually worked perfectly fine for me for years on my ST40 kit, like I said there seem to be some slight differences in bores/sizes when cross used on Stoptech parts vs Brembos.

I ordered the dustboot replacement for the 2 Girodisc ones that pulled out (on different calipers) from FCP, they seem to be Brembo, yet to install, see how they last. Rest that are not pulling out from Girodisc have been through a few events and seem to be holding up.

For all the above I ran cooling ducts and 1mm Ti shims.

Now this won't answer your question directly for how to solve it, but I'll go back to my comments I made in other 996 threads, and it's related to "a" solution. 996 front kit with "stock" 345 rotors is not a great setup for track. Take FCP out of it and it actually kinda sucks. Main part of a brake system for proper heat evacuation is the rotor... the semi floating design of these 345 rotors actually makes it hard to pull air between the hat and both ring surfaces, and the hat/hat mounting actually kind of obstructs air flow (pic 4).

Also don't forget that in terms of heat capacity the 996 front pad is just BARELY larger in size vs the stock m3 pad (see pics 5,6,7), so there is a lot of energy that little pad takes in, combined with poor rotor flow and heat evacuation, well, it transfers on down to the caliper, even with Ti shims slowing down the heat xfer.

So bullet proofing the dust boots and seals on their own I don't think will solve the problem, the problem is somewhat inherent in the holistic system where all components contribute to the problem.

I am also not a fan of frequent rebuilds. On a dual duty car that swaps pads track to street I think dust boots are essential. Track time generates a lot of metal shavings that you would then compress into the pistons/bores as you put in fuller/less worn street pads. At some point the bores and pistons will get scored and the seals themselves could get cut.

So I don't have a solution, but some combination of resistance to heat xfer in forms of shims, potentially swapping out Alu pistons, getting a "proper" rotor design that actually cools ala Girodisc, ducts to cool the pads, Brembo or Girodisc dustboots is in the vicinity.

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[cobra]

On motorcycles, dust boots are not used - likely due to space constraints since the calipers have to be as narrow as possible to clear the wheels. But... they do have a dual seal: the inner square cross-section brake fluid seal and an outer dust seal. I believe this is to prevent oils, dirt/debris, chemicals from coming into contact with the critical inner seal which is made from EPDM. EPDM is great at dealing with brake fluid and water, but will degrade quickly under exposure to unforeseen chemicals and I am sure you want to keep grit out of that interface as the seal rocks back and forth during each brake application.

That said, the pistons on motorcycles do not hold up well. For decades they were made from chromed steel (like stock M3 pistons) and they will eventually get pitted and rusty. This is fine until you go to change brake pads and have to push a nasty pitted piston past the seal, causing a leak or friction. Newer/nicer brakes use hard anodized aluminum pistons that won't corrode and hold up a lot better with this arrangement.

On a car, boots are used and are super nice because they can seal the entire piston from the elements, hopefully keeping the entire thing clean. As long as the boots are good, when it comes time to change pads you just push them back in and you're good to go. I can see the argument for not needing a boot on a race/track caliper where it might get rebuilt frequently, but I would absolutely want them on a street driven car for longevity. The OEM's don't put them there for funzies.

[Casa de Mesa]

Got it. This makes more sense now.

I enjoy the root cause analysis part sometimes more than the actual solution.

[Obioban]

I guess where I'm coming from... If it seized suddenly (which it appeared to), that seems suspect. I feel like a sticking piston due to dirt or corrosion should have revealed itself long before via other tell tale signs (car felt down on power, smelling the stuck pad on the drive to the track, a dusty wheel when all others were clean, a scorched rotor). Would be interested to see if the pad is severely worn on one end, especially compared to the same same caliper on other side of the car.

What I'm curious about is the suddenly frozen part.

And I'm still in for the solution!

So, it was worn out more (~double the wear of the other side).

The previous lap he thought he maybe smelled hot brakes, but they felt fine and the smell went away so he assumed it to be someone else's car.

The session before he noticed the car was pulling to the right under braking.

So, it wasn't exactly spontaneous-- but it did suddenly not have braking (probably faded on that side) on the lap in the video.

[Casa de Mesa]

He hasn't pulled them apart yet to inspect, but that seems like the most likely culprit to me.

I guess where I'm coming from... If it seized suddenly (which it appeared to), that seems suspect. I feel like a sticking piston due to dirt or corrosion should have revealed itself long before via other tell tale signs (car felt down on power, smelling the stuck pad on the drive to the track, a dusty wheel when all others were clean, a scorched rotor). Would be interested to see if the pad is severely worn on one end, especially compared to the same same caliper on other side of the car.

What I'm curious about is the suddenly frozen part.

And I'm still in for the solution!

[Obioban]

First, glad he and the car are ok.

Second, did it seize because of debris for sure? Seems unlikely... but maybe not.

To be clear, I'm in for the solution despite my prior post.

He hasn't pulled them apart yet to inspect, but that seems like the most likely culprit to me.

[Casa de Mesa]

First, glad he and the car are ok.

Second, did it seize because of debris for sure? Seems unlikely... but maybe not.

To be clear, I'm in for the solution despite my prior post.

[Obioban]

Calipers seizing up from crap in them is no joke-- this really took center stage for me at this moment because a friend of mine had a front 996 caliper seize up on him last weekend on track, at ~130mph:

Could have gone worse

... I'd still like to keep crap out of mine, which means dust seals.

[Obioban]

If it's a track only car, sure, no seals and frequent rebuilds may be fine.

... but if it's also a DD, that quickly isn't tenable.

I ran the same dust seals (whatever they came with stock) on my Brembo GTs for 13 years/20,000 track miles, and never had them fail. So, this isn't an unobtainable goal.