Sudden hard brake pedal on track

beanie

beanie

Active member
 Bournemouth/Turin
Yesterday evening I did a 15 minute track session but towards the end I noticed the pedal went very hard. Strangely, and thankfully, braking power seemed unaffected. I backed off and went easy for the remaining time and on the drive home everything was back to normal.

I’ve had the pedal go long before and that made sense, but going hard and with no reduced performance (easy now!) seemed counter intuitive. Most info I can find about hard pedals seems to point to vacuum/servo issues but as this coincided with track use I’m not convinced that’s the case.

Is this just a symptom of a non track setup being pushed too far (fried pads? The edges certainly look a little flaky now) or could it be a vacuum issue?

Brakes are stock apart from high temp fluid as I’m not a regular track goer but my discs are getting worn and the pads are probably dead now so I’m starting to eye up some new gear.
 
Sounds like the brake booster to me. Presume you haven't coded out the 'assisted braking'?
I coded mine out long before I started going out on track so can't tell you what it feels like but I certainly remember a few on here complaining about it and giving the effect you describe before they coded it out. Could be because you were pushing more to the end of the session you were activating it more as opposed to the start of the session?
 
Brake assist is still present but I didn’t notice any extra random moments of assist at any point. Having had a long pedal before I was only 7\10ths on the brakes apart from the odd corner here and there.

I had a cold start issue a few years back that was eventually traced to the check valve in the brake booster system. It was replaced but perhaps it’s gummed up again. The car does ‘breathe’ a bit of oil vapor back in to the air box and I think that was probably the cause.

Just to add, once the pedal went hard it remained that way regardless of braking force or speed of application. It wasn’t until it cooled down that it returned to normal.
 
Maybe not the brake assist then but certainly points to something with the brake booster / master cylinder circuit. Not something I've come across personally, hopefully someone else can help.
 
Wild stab in the dark, but could it be all the tolerances closing up in the calliper due to the heat. So the pad doesn't back off so far and therefore there is no 'slack' to take up when you hit the pedal?
 
Thanks guys. I’m a little stumped as, given what I’ve read, I’d expect a more solid pedal to come with reduced braking performance meaning a possible cooked/glazed pad. There’s a chance that had I continued to push, that would have happened and the answer would be clear. I’m still clinging to the hope it is just pads and nothing more complicated or expensive!

I’ll give the oil separator and the check valve a look over and clean but for better or worse, it’s not a situation I expect to be able to replicate unless I’m back on track. I suppose it’s a legitimate reason to do so :lol:

I’m going to have to measure the front discs and see what’s left as there’s a fair lip on them now. Can’t remember when they were last changed but it’s been a good many years and miles since. If they and the pads are shot, then perhaps it’s time to look at other options. A bbk is probably overkill, but I do fancy doing a little more track work and having to drive around the brakes every time is going to get tiresome pretty quickly.
 
Nah, I've seen it before.. you can reproduce the effect by being on full throttle and then rapidly coming off trottle.. the brake booster gets primed for an emergency stop. Lots of fun.. lol :thumbsup:

Edit: I just re-read your initial post.. so the brake pedal remained ..hard.. for the rest of the session?? Than that's not what I thought it was.. I'd pull the master and check everything.. soft pedal is easy, you just overheated the brakes.
Stiff pedal that changes enroute??? Weird.. I do know the emergency brake assist pre primes the brake system vacuum if you let off quickly, but it doesn't remain in that state...
 
Sorry, reading back over my posts perhaps it wasn’t clear how it went.

I’ve had the brake assist happen on the road and this was different. The pedal went very firm and the travel was greatly reduced. Tbh, it felt quite good, but it didn’t feel wise to carry on regardless!

I only went round a few corners with the pedal like this but during that time the pedal stayed hard even with reduced braking effort. It seemed to go back to normal within a lap or so of cooling and light braking and was fine again for the last few minutes of the session.

Took the car out for a gentle run yesterday and all is normal, if anything the brakes feel a little softer than usual but no strange noises or behavior
 
The brake booster doesn’t make the pedal feel hard (at least it never did on mine during several dozens of track days over more than a decade of ownership). It augments the braking effort so more braking effect is generated for a given pedal travel. The consequent unpredictable braking effort for a given pedal application, especially during heel-and-toe, led to me coding it out.
I have had the brake pedal go hard during track days, exactly as you describe, and it seemed to be associated with the brakes overheating. I can’t remember the exact setup I had at that time, but I was definitely running track orientated pads and fluid. I don’t remember it happening after I changed to solid brass brake bushings and added brake cooling ducts, so perhaps it’s to do with the OEM rubber bushings overheating and becoming sticky. It certainly hasn’t happened with the AP kit I’m running now.
 
I found the same with the brake assist. A sharper reacting but not harder pedal.

I’d love a set of APs, total overkill for my use but they’d certainly deal with anything I’m likely to throw at them. I was also looking at the BMW performance calipers in order to keep the standard discs/reduce costs but also allow for a larger pad with a bit more capacity to take occasional track sessions. Pad choice no doubt has more bearing on heat resistance but they look nice and I’m a tart!

First step will be to change over to a more robust pad, something a bit more suited to track work without being a chore in normal road use. If that doesn’t work some kind of additional cooling would hopefully see things resolved.
 
It’s worth looking into solid brake bushings too. They’re inexpensive and offer no disadvantage (just lubricate the slide pins regularly with an appropriate anti-seized compound).
 
I’ve looked in to them in the past and came close to buying them but the odd negative report and the extra maintenance ended up putting me off.

Which ones did you go for and roughly how often do you get in there and lube things up?!
 
I can’t remember where I got them now, it was a long time ago! I was doing lots of track days every year when I was back in the UK and changing pads several times a year, so it wasn’t much additional maintenance for me.
 
Ideally I’d do the same with the pads but I tend to decide on a whim so there’s never enough time to do anything other than check the fluids and fill the tank!

I’m tempted, but not sure if the additional faff is something that I’ll tire of.
 
I’m not sure if there’s sufficient benefit to be gained to warrant the faff either. With OEM callipers and discs, track orientated pads and fluid, solid brake bushings and additional cooling, the brakes (almost) always worked fairly well, but there remained an inconsistency to pedal feel over the course of a session as the brakes retained more and more heat. (I lost all braking effort only once, but that was braking from over 100mph into a 90 degree left. Fortunately, I had some run off…)
Since fitting the AP BBK front and rear and coding out the emergency brake booster, the brakes are consistently excellent, even in 35 degree Australian sunshine.
My KW suspension setup means that the car doesn’t pitch forward under heavy braking as it did with the OEM springs / OEM dampers or Eibach springs / OEM dampers. That also seems to have significantly shortened braking distances.
 
Thanks for the info BMWZ4MC.

As much as it may end up costing a little more in the long run, I think an incremental upgrade plan is probably best for me. I’ll skip the brass bits for now and see how it goes.
 
Guys, I having same Issue with my brakes same scenario as Vanne described ( being on full throttle and then rapidly coming off throttle ) , in my case It start happening right after upgrading to GPOWER super charger and only on the race track no way to reproduce it on the street driving even with very aggressive manipulations, also I have 4.10 diff LOL -> I guess its even worse scenario in my case.
 
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