Spongy brakes randomly during really hard drive
 

Was doing some pretty intense backroad driving, and all of sudden the top of my pedal got squishy, and it hasn’t gone away since. Brakes seem to work okay but all top of pedal bite / confidence is gone. I also find that if I’m driving 40-60 mph, and I just slightly graze the brakes to slow down a bit slowly, the pedal sinks just a bit and stops - maybe half to 1 centimeter. Top of pedal is just spongy now, and I drove a friend’s 2006 back to back to confirm - it’s definitely way squishier.

Been a couple weekends now.

I last changed to Motul RBF600 about 2.7k miles ago (a few months ago), most of which have been hard backroad driving. Not sure how old pads / rotors are.

Could it be that I cooked the RBF600 on a backroad drive? Or I damaged / overheated pads? Super worried that I busted my brake master cylinder somehow.

2004 AP2 with 50ishk miles. Just got it in June.

Any ideas? Should I start with a brake bleed and go from there?

Check the caliper slides and pads/hardware.

something is stuck

After you check the above, re-bleed your system for air in your lines.

My technique for a full flush:
- Gravity bleed each corner - Pass Front first / Driver Front second / both rears at the same time - (assumes left hand drive car)
- Pull fuse for brake lights -> crack open brake reservoir -> pin your brake pedal with a stick and let sit over night
- re-install brake light fuse
- Go for a drive, slam on brakes a few times - enough to actively engage ABS system each time
- Repeat bleed procedure

That last piece on actively engaging your ABS system is what will get the last bits of air out that might be trapped in your system.

Hi guys, so I’m still having this issue and really frustrated.

Here’s what I’ve done:
1. fully flushed with gravity bleed using @Billman250 ’s procedure. This helped maybe 10%.
2. Pinned pedal overnight with a jackstand also suggested somewhere by Billman. Helped maybe 20%.
3. did a 2 person bleed (not full flush). 3 pumps per corner counterclockwise starting at RF, did all corners 3x. Helped a bit more.
4. Had Joey (an s2000 specialist: @itS2Krazy ), change my OEM pads to Endless MX72 with brand new centric rotors, and did a whole flush using a pressure bleeder. He also greased the slider pins.
Now the pads stop harder than oem once you dig into the pedal, but the bite is still nonexistent.

All bleeds and flushes used Motul RBF600.

It’s still not nearly as good as before. All the bite is gone and pedal feels like theres 1-2 cm of mushiness before it gets hard.

As a reminder of the symptoms:
1. very little bite feel or pedal stiffness at the top. occasionally it appears again when braking, and I’ll feel like the brakes are magically fixed, and then it goes away again. At a stoplight, it is the exact same consistent mushiness at all stops. The pedal does not sink past the hard point, the hard point is just lower than normal.
2. sometimes at speeds anywhere from 30-80, I’ll be just grazing the brakes. maybe 2-5% brake, and out of nowhere it feels like pedal gets pulled down a bit more, maybe half a centimeter. It feels as if the pad shrunk. Happens maybe 1-2 times per 2 miles.
3. I feel like i have no brake confidence. I used to tell my friends how much I loved the s2000 brake feel, now I’m almost saddened by how uninspiring it is in comparison to the rest of the car. I can’t do a single instance of braking without paying attention to it either. I drove a friend’s s2000 with stoptech pads and he has tremendous bite, and when he drives my car he agrees: “there’s something just missing from the top of your brake pedal”

This started in the middle of an intense and long backroad drive, just 3000 miles after my last RBF600 gravity bleeder. Brakes felt fantastic, then out of nowhere it just sunk a bit, and the point where the pedal becomes firm has been perpetually lowered.

What could I look into here? Is this an early sign of a failing brake master? Do I have some hidden air that I can't get out?

EDIT: I also triggered ABS on the road before every one of the bleeds except the first gravity one.

HELP: Slight sponginess at top of pedal, can't figure it out
 

Hi y'all, reposting because I'm really struggling here and a bit worried my brake master cylinder is going out.

About 6-8 weeks ago, I was on a pretty intense backroad drive, and out of nowhere the top of my brake pedal just got squishy. I had phenomenal brake feel before this, and my RBF600 fluid was maybe 2000 miles old at that time (I gravity bled it a few months before). I bought this 50k mile AP2 about 6 months ago.

Here's a detailed explanation of my symptoms:
1. very little bite feel or pedal stiffness at the top. occasionally it appears again when braking, and I’ll feel like the brakes are magically fixed, and then it goes away again. At a stoplight, it is the exact same consistent mushiness at all stops. The pedal does not sink past the hard point, the hard point is just lower than normal.
2. sometimes at speeds anywhere from 30-80, I’ll be just grazing the brakes. maybe 2-5% brake, and out of nowhere it feels like pedal gets pulled down a bit more, maybe half a centimeter. It feels as if the pad shrunk. Happens maybe 1-2 times per few miles. This is what makes me feel like my master is going bad.
3. I drove a friend’s s2000 with stoptech pads and he has tremendous bite, just like what mine used to feel like, and when he drives my car he agrees: “there’s something just missing from the top of your brake pedal”. The point where the pedal becomes firm has been perpetually lowered.
4. When I dig into the pedal hard, I have phenomenal brake response. I sort of have just trained myself to believe the pedal engages a centimeter lower now. It's just jarring because it used to not be that way 2 months ago, and occasionally the bite returns temporarily.

Here’s what I’ve done to fix:
1. fully flushed with gravity bleed using @Billman250 ’s procedure. This helped maybe 10%.
2. pumped pedal in garage like 10x, then pinned pedal down overnight with a jackstand also suggested somewhere by Billman. Helped maybe 20%.
3. did a 2 person bleed (not a full flush). 3 pumps per corner counterclockwise starting at RF, did all corners 3x. I think I saw some bubbles come out from the fronts, and this helped the most, but still not near as good as before.
4. Had Joey (an s2000 specialist: @itS2Krazy), change my dying OEM pads to Endless MX72 with brand new centric rotors, and did a whole flush using a pressure bleeder. He also greased the slider pins. Now the pads stop harder than oem once you dig into the pedal, but the initial bite is still nonexistent. It’s still not nearly as good as before. All the bite is gone and pedal feels like theres 1-2 cm of mushiness before it gets hard.
I think it actually feels a little worse than Step 3 now, but that might be due to pads needing proper bed-in? Not sure. Maybe feels about the same after bedding them in. Just feels bad.

All bleeds and flushes used Motul RBF600. I also triggered ABS on the road before every one of the bleeds except the first gravity one. I must've triggered it like 15 times by now (it is super wet outside in the bay area, california, so easy to trigger ABS).

I feel like have no brake confidence. I used to tell my friends how much I loved the s2000 brake feel, now I’m almost saddened by how uninspiring it is in comparison to the rest of the car. I can’t do a single instance of braking without paying attention to it either.

What can I do next?
- is this an early brake master cylinder failure symptom? ('05 uses AP1 BMC, and those are discontinued
- do I just have some extremely stubborn air somewhere?
- could my brake hoses be expanding on initial pedal compression? or failing in general?

The fact that every bleed has helped a little bit is giving me hope that it's just some really stubborn air. But not sure what to do or how to even verify this.

First you can't throw too much shade at a 22 year old braking system. 50k miles on a s2000 to me means that its sat for long periods of time and seals deteriorate..

The top of the pedal is all booster. If there's a change at the first 30% of pedal throw that's usually a tell tale sign something funny is going on with the booster.
The chance of air in your system is low with all the bleeding. To me it sounds like the diaphragm inside the booster is leaking. If the booster completely fails then you will have a very ridged brake pedal. If its a small vac leak then you will have weird changes to intermittent brake feel.
Good news is they're easy enough to change and cheap enough to buy.

For a test you can take off the booster vacuum line and cap both the intake manifold rubber line and the booster. It should have a constant heavy pedal feel, (No vacuum assist). Go and drive around and get some heat into the pads. If you notice anything weird as in changes with the pedal modulation then its the master. If it feels consistent after driving around for a bit and braking a lot to build a feel of the pedal then its your booster causing issues.

PS also properly bed in those pads. That will help with proper feel.

Thank you!! Will try this out. It’s safe to drive without the booster hooked up to vacuum right?

And yep, I think I’ve bedded them in now - feels exactly the same as before I swapped the pads lol.

Also for what it’s worth:
- after turning off the engine the pedal becomes rock hard after 3-4 presses
- if i keep my foot on the brake when starting the engine, it immediately sinks a bit. I think it sinks to a slightly lower point in the pedal than it used to, but just by maybe 0.5-1cm lower.

Not sure if either of those indicate that the booster is working properly

That is normal operation with a brake booster.

Your rear brake pads...

Is the alignment slot on the caliper piston aligned properly with the brake pad's alignment post?