Pad Knockback or bad BMC? Pedal going long again after flush n ABS bleed

[BigRussia]

Sorry for the long post, thanks in advance to anyone who reads and can help me resolve this lingering brake issue!

Ive been chasing on track braking performance issues with a long/spongy pedal for the better part of this year. I was getting very long brake pedal travel in sessions, and the pedal was very spongy even after on the drive home my previous track day in May. Running Castrol SRF, CSL/ZCP brake setup, and G-LOC track pads. The brake master cylinder showed no signed of leaks and with engine off the pedal wouldnt keep going down to the floor, but I knew the brake fluid was in need of a flush and the ABS likely had air in it so over the summer break I did the following to address it in hopes of resolving the long pedal and lack of braking confidence issue in time for returning to the track in the fall/winter (too hot in SFL for summer tracking anyway).

Over summer break:
-Bled ABS using INPA at all 4 corners: did have air
-Flushed system brake fluid with fresh Castrol SRF 1L bottle: old fluid was probably degraded and with moisture, hadn't fully flushed in a couple years
(both of the above done with Motive Power bleeder)
-Installed BBK calipers just as an upgrade for bettering braking gain and heat capacity: new Freakyparts Megane RS front with rear 996 Brembos
-New G-LOC track pads to go with new bigger calipers. R16 front R10 rear compounds
Still running OEM CSL/ZCP rotors front and rear

After the above were installed with new street Akebono pads, the pedal was very firm and felt very good. I was relieved and figured the pedal issue should be resolved and that the BMC was not faulty.
1st track day with the new setup yesterday at Homestead Miami Speedway road course and halfway through the first session pedal gets long again, at one point so long I was worried the master had surely gone bad!
But then thinking maybe its pad knockback w the now fixed calipers I started left foot tapping the pedal before the hard braking events and yeah it would firm up the pedal once I got on it into the braking zone. But again couldn't that also be a symptom of air getting sucked in by a bad seal or something? Basically I'm thinking that compressing the air in the system w the left foot taps would feel the same as pushing the pistons back if there was knockback? Another clue is the pedal was spongy even after the long main front straight of HMS, where I'd assume there wouldn't have been much knockback and I wouldn't let foot tap before Turn 1, and basically couldn't even get the car to stop as quickly, felt like it was still on throttle even!

This is now a lingering issue and dont know what the cause is other than the two items above, I dont want to spend the time and money replacing the BMC if it doesnt resolve the issue and really it's just pad knockback, or something else with the car. Even now the next day with the engine off sitting in the car and pressing the brake pedal it stays firm and doesnt keep going down to the floor, which points to the master being OK. Or is it something else system related that could be causing this? Maybe a vacuum leak at the booster? Other factors being wheel bearings, while I dont know their history and havent been replaced recently or in my ownership, there is no wheel play or clicking/noise from them that I can tell.

Thanks in advance for any help diagnosing this! I really want to make the most of the winter track season this year!

[foolio]

I don’t know what your problem is, but I can share a couple of thoughts. First of all, for me, pad knockback is instantly solved with a tap of the brake as you mentioned. Does it solve it instantly, for you? If it doesn’t, then I don’t think it’s pad knockback. Once you get the pads back on the rotor, you should have 100% breaking with a good pedal feel. Besides INPA bleeding, you also said you bled again. What method did you use? I always have good results with a Motive Pressure Bleeder. While SRF has a high boiling point, it’s not my favorite for pedal feel…not that it’s bad, I do use it occasionally but I like something a little harder feel.

Lastly, I don’t have the details, but, there has been talk about people chasing pedal feel issues finally resolving it with a new booster. Not common, but heard of. Might see what you can dig up on that.

[BigRussia]

I don’t know what your problem is, but I can share a couple of thoughts. First of all, for me, pad knockback is instantly solved with a tap of the brake as you mentioned. Does it solve it instantly, for you? If it doesn’t, then I don’t think it’s pad knockback. Once you get the pads back on the rotor, you should have 100% breaking with a good pedal feel. Besides INPA bleeding, you also said you bled again. What method did you use? I always have good results with a Motive Pressure Bleeder. While SRF has a high boiling point, it’s not my favorite for pedal feel…not that it’s bad, I do use it occasionally but I like something a little harder feel.

Lastly, I don’t have the details, but, there has been talk about people chasing pedal feel issues finally resolving it with a new booster. Not common, but heard of. Might see what you can dig up on that.

Yeah when I would left foot tap the brake pedal before approaching a braking zone, the pedal would be firmer when I actually got into it with my right foot. I was thinking but then why on Turn 1 after the main straight it would also be spongy, but realized well could've got pad knockback on the last corner before the straight. But if this is really what pad knockback is, this seems excessive work... having to left foot tap before every single braking event? Is this really what everyone else with fixed calipers is doing? Are pro drivers in race cars doing this multiple times a lap, multiple laps throughout an entire race?

And as for the bleeding of the ABS and calipers I used a Motive Power Bleeder with the dry method And yeah I had a buddy tell me the same about SRF, that they didnt like how it felt and preferred something harder feeling. Being that it only comes in expensive 1L bottles, I had been thinking maybe changing to something higher temp and harder, that also comes in smaller bottles for service bleeding.

I at first considered could be a bad booster, since maybe a bad one can suck in air? But dont know how to test that, other than when I turn on the car the pedal does go down so seems to be working at least. Also with how boosters work I figure during a track session where your engine is basically never making vacuum the booster would be working less/not at all and thus pedal be less boosted and more firm, rather than what I am experiencing.. ugh so many variables!

[S54330Ci]

. . . having to left foot tap before every single braking event? Is this really what everyone else with fixed calipers is doing? Are pro drivers in race cars doing this multiple times a lap, multiple laps throughout an entire race?

Yes - some do.

And as for the bleeding of the ABS and calipers I used a Motive Power Bleeder with the dry method And yeah I had a buddy tell me the same about SRF, that they didnt like how it felt and preferred something harder feeling. Being that it only comes in expensive 1L bottles, I had been thinking maybe changing to something higher temp and harder, that also comes in smaller bottles for service bleeding.

I've had great luck with both Motul RBF660 and ATE200. Never an issue with boiling the fluid on the track.

I at first considered could be a bad booster, since maybe a bad one can suck in air? But dont know how to test that, other than when I turn on the car the pedal does go down so seems to be working at least. Also with how boosters work I figure during a track session where your engine is basically never making vacuum the booster would be working less/not at all and thus pedal be less boosted and more firm, rather than what I am experiencing.. ugh so many variables!

Vacuum brake boosters can't introduce air into the hydraulic system as it is a closed system. Driving under track conditions do not affect how your brake booster works, otherwise all race teams would experience the same issue.

I really doubt you are experiencing knock-back. IMO you still have air in your brake lines. You may need to bleed multiple times to get all the air out.

[BigRussia]

I really doubt you are experiencing knock-back. IMO you still have air in your brake lines. You may need to bleed multiple times to get all the air out.

Yeah I'm leaning and hoping that is the case, even though I bled a decent amount of fluid from the ABS doing multiple INPA triggers at each corner (a few more at the fronts vs the rears). I want to believe the air that may have got reintroduced into the system was from braking hard triggering the ABS in the first session at the track, but could there really still have been that much air in the ABS to then get cycled into the system? Again the pedal was nice and firm before the track day, after the summer brakes refresh.

But yeah to rule it out I am going to bleed the ABS and all 4 corners again ASAP hopefully over the long weekend, with a fresh unopened bottle of SRF I have. I have another track day coming up soon too that will test it. It's Sebring at night, so def want to have the brakes at their best if I can.

[BigRussia]

Did a full bleed again over the long weekend. Triggered the ABS using INPA at each corner 3-5 times per brembo caliper side, then bleed the corner ~200 before moving to the next. All done with Motive Power Bleeder using dry method, and a one way bleed valve at the bleed nipple, ran through ~900ml of brand new SRF bottle. To my dismay I didnt really see any air come out during the ABS bleed triggers or from the calipers during the longer 200ml bleeds... Test driving after with the same G-LOC track pads the pedal seems just normal, not too firm as when first installed BBK (but was with Akebono ceramic pads then), but not too soft to where its not confident on the street. Wont know for sure till next track day at Sebring in early December.

[CrookedCommie]

Switch to a Ferodo 1.11 or 3.12 pad, maybe it's the G-LOC compound. You mentioned the pedal felt fine with a street pad. Might be a good idea to switch to a new rotor too, eliminate any chance of previous pad deposit causing issues.

[BigRussia]

Switch to a Ferodo 1.11 or 3.12 pad, maybe it's the G-LOC compound. You mentioned the pedal felt fine with a street pad. Might be a good idea to switch to a new rotor too, eliminate any chance of previous pad deposit causing issues.

Yeah, I too was thinking could it be that the G-LOC pads just have alot of compressibility? But also realized its not fair I was comparing them to the street Akebono pads since ceramics are inherently stiffer pads by their nature lol. Even then i feel the amount of pedal sponge/sinking i was getting is more than would be acceptable for just soft track pads?

I’ve heard great things on the DS1.11s, a buddy recommended switching to them as well. I’ll be looking into them for my next pads when I replace the CSL rotors once finished off with new Girodisc ones.

Oh forgot to update; I checked my wheel bearings as well during the bleed, and none of them had any play (shaked back and forth, and side to side 12 n 6, shockingly no play whatsoever). Kinda surprised how these wheel bearings have been holding up with the 5 years I've been tracking and how hot my brakes get lol, BMW M made some robust wheel bearings it seems? I have no idea if a PO replaced them but no way theyre the original factory ones.

[bigjae46]

Couple of things..

The symptoms of a bad brake booster are a hissing sound when you press the brake pedal, erratic engine idle when not on the brakes, engine lean codes, and/or an extremely stiff pedal. Sometimes, people will mistake a slightly stiffer pedal with spongy.

Sliding calipers don't experience pad knockback.

A low temp street pad works differently than a high temp race pad. A low temp pad simply works on pressure and friction and creates mostly brake dust.

A high temp pad needs heat and adheres to the rotor when pressed against it. The dust you see on your wheels is mostly rotor material. Which is why it is important to bed in high temp pads to optimize braking performance. If you don't bed in high temp pads then most pads will wear out extremely quickly and also chew up the rotors.

Using a street pad on the track will likely overheat the pad. Once the pad is overheated, a couple of things can happen. The pad material delaminates from the backing plate, the pad disintegrates or (most common) the pad becomes hard like a rock and has zero bite.

I'd bet that your pads were cooked which was causing the spongy feeling and bad braking.

Fixed calipers won't bite like a slider. A sliding caliper has a twist that has to be taken out and then it bites. That will create a feeling of the brakes being more sensitive or having more bite. A fixed caliper will be far easier to modulate and operate smoothly.

[BigRussia]

Drove up to Sebring Intl Raceway this weekend, for a 2.5hr open track session at night on Saturday. Brake pedal didnt feel any better or worse, still inconsistent. Sometimes it's decently firm and 'normal' travel, then other times its more spongy/sinking, to where i would have to resort to left foot tap again before harder braking zones.. but then like a lap later it would be firmer again. So this is after the re-bleed of the ABS and calipers with a whole new 1L of SRF. Not surprising there wasnt really improvement since I didnt see much if any air come out of the bleeders, but at least can rule out any air remaining in the system. I know SRF isn't the most firm fluid but it cant just be the fluid getting hot, I have Ti heatshields on the front calipers now too and the fluid is brand new.

But when I got home after the 3 hour drive from Sebring (had to fix a flat on the way too), I popped my hood and then heard a hissing sound coming from the brake master/booster area!

https://youtube.com/shorts/5HkEY2I5a4g?feature=share

I dont recall ever hearing anything like this before, and I regularly pop the hood when I get in my garage after even a short drive... I'm hoping this is related to my inconsistent brake pedal issue. My first thoughts were maybe a bad booster vacuum check valve? Nothing looks very brittle or worn, and the hoses even look ok. But what are you guys' thoughts? Or is this potentially a whole new issue? The brake pedal right now with the car off is still firm, and when pressing it i dont hear a hiss. And from what Ive read my symptoms dont look like what people say with bad booster vacuum check valves... maybe I'm at the point to just suck it up and get a whole new brake master?