Kailuahawaii

Brake drag noted driving as usual. Jacked car and by then wheels turn ok but L and R front are (really) hot. Car is manual.


Same thing happened (now trice in a row) after ~20min stop and go traffic. Did not happen when I test drove shorter duration...


No recent battery change / brake changes (they have wear but are in specs, stock). Again the wheels turn (I am not sure how freely they are to turn with a C4 - they are tunable but feel a bit stiff and did not notably improve with brake fluid release from caliper??) when jacked but by then the car had probably cooled down some.(?).


When it happened, I could accelerate to overcome braking, taping the brakes would (on and off) yield freeing of this effect... Allowed me to get home without a fire I guess It does not help to pull up on brake pedal / brake lights are not on when this happens.


Car has ~45K, owned it (??) 4-5 years, ran durametrics (got 'pro' version) and see a ride height control unit error (U0129) but nothing else that seemed relevant (some AC can not communicate with PSM ect...).


Removed PSM fuses C8, C9 and C10 - idiot lights came on and braking issue as well (after 20min drive)... Braking issue seems to come on mildly (and get moderate) without me pressing brake, just notice the car simply wanting to stop when I lift off the gas!). Wheels have not locked up.


Brake fluid looks clean, level appears unchanged (~max line) from last time I checked >6 months ago.


Reading here and in general -


Could be master cylinder failure of some type where the brake fluid return ends up blocked (why only front two wheels) when heated? Is that something that really makes sense / plausible??


Could of course be ABS being very weird... This feels quite nebulous to me!


Recommendations for what to look for as I try to recreate situation with durametric plugged in? (wheel speed sensors / something else?). I will try to track the durametric value on brake applied pressure to see if it provides a reading despite pedal not being pressed when event happens.


Yeah, dealer is plan B but would love to actually understand the system as much as possibly save some $$. Forced to drive the 955TT in means time but really have mixed feeling about having sold my '85 for the 997 (other than the AC upgrade which is a no brainier!)


Am I on the right track? Missing something much more in my face!? Would you guy's favor mechanical vs electronics? (is it realistic for a sensor - that feeds psm / abs to fail when warm and behave again when cooler?).


Looking forward to insights from the collective experience . Mahalo from Hawaii

8x57IRS

First I would look at the pads and general mounting situation, you also need to check whether the pistons are completely free. When did you change your fluid last time?

Petza914

It's either the master cylinder failed, the caliper pistons sticking, or the rubber line have internally swollen and won't release the pressure quickly enough. Since you say tapping the pedal improves the situation temporarily, I'd say it's the MC.

MrMoose

If it's both front tires I think that rules out the hoses, since it's unlikely both rubber lines would've failed at the same time.

My first guess would be the brake booster, if not that then the MC. When it happens, try disconnecting and plugging the hose to the brake booster (the brakes will still work but you won't have power assist, so be careful). If that fixes the problem your booster has gone bad.

yelcab

This is what happened to various vehicles that I have owned and worked on.

1. Brake booster. Newly rebuilt ones failed right off the box. A factory new booster solved that problem. Yours may have failed.
2. Brake master cylinder. One circuit drives the entire front end. Both locking up says this is most likely the issue.
3. ABS valve unit. Strange things happened to ABS units as it ages.

Kailuahawaii

So a MC could fail in way that makes both front brakes apply - it this a replace and see if it gets fixed or not?

(general condition of calipers and pads appears good (I did not pull pads but they are not thin and all appear the same thickness, I was under impression I should change the pin if I pull the pads).

As for the booster failing, would pulling power to it be the way to test this theory? (some have mentioned not being able to brake much when theirs failed, will have to try as slowest speed Or could booster failure be more physical / mechanical and would need to replace to know?

Thanks for pointers, I will try durametric reads today if I can get it to do this again safely but feel of the replies is more towards MC which would not really be picked up with it...

MrMoose

The booster doesn't have power, it works on vacuum. Disconnect the vacuum hose from the booster and plug the open end of the hose (not the booster, you can leave that open: the hose). You'll still have brakes but no power assist.

If that fixes the problem your booster has gone bad.

Kailuahawaii

Aloha Mr Moose - yeah, I can be a bit thick I see the vacuum line now... Will try as well! Thank you

saxonite

Sounds like the pistons are locking up and not fully releasing back towards the caliper after each brake event. I've had this happen on my BMW but it was just one caliper with the issue. Being both from calipers are doing the same thing, it makes me think the proportion valve is faulty. Dirt could have gotten in a clogged it. I did have to clean out one of the bleeder valves last spring because it was clogged with something.

Kailuahawaii

Gentle trial run with booster disconnected - front rotors hot / rears lukewarm. Same (gentle) drive with booster connected done earlier - front rotors far far too hot to touch / rear lukewarm. (did not get the car to brake on it's own as prior perhaps due to shorter drive).

Should I assume a bad master cylinder (with effect aggravated when booster is connected)?

(the brake fluid looks clean, was replaced ~4yrs / 25k miles ago, certainly due now but would not have anticipated crud building up to block stuff but possible I suppose).

MrMoose

Well, the front brakes will usually be pretty hot even after a few stops, so I'd make sure they're actually dragging and that it isn't just normal brake heat.

Maybe try a longer test to make sure you can replicate the braking on its own? Or try driving with the booster connected until the problem happens, *then* disconnect the booster and see if it goes away?

In thinking about it, you can probably just put a temporary clamp on the booster hose, that'll be easier than disconnecting it.

MrMoose

Also, I doubt it's your current problem, but 4 years is really too long for brake fluid. Should be done every 2-3 years, Porsche says 2.

Kailuahawaii

Same 'auto braking' effect without booster, this time within first mile (feels like brakes applied at ~30%, steering wheel shake at 35mph, with no side pulling, again both fronts - maybe some relief with tapping brakes but not sure that was was made it lighten up some to perhaps 15% on way back home).

Ordered master cylinder, it will be here next week and will share update, seems an unusual failure. I will get to also flush system and will try the durametric for bleeding ABS (hopefully effective).

MrMoose

Yeah, MC is the next thing I'd try. Fortunately it's fairly easy to replace on these cars. Take out the whole big rear plastic trim piece in the frunk to access it.

Get a lot of brake fluid. When I replaced my MC I pressure bled normally and still had a soft pedal. I had to follow the second pressure bleed instructions in the service manual, pushing and holding the pedal to the floor a few times for each wheel while the bleeder was open. That got it.

I did not need to use the Durametric ABS bleed procedure, which is good because when I tried it I found out it wasn't working! I reported the bug to Durametric and it's supposedly been fixed in 6.6.1.6 and later, though I haven't had a chance to test it yet.

Bruce In Philly

2009 C2S 184K miles

My vote is the pistons are sticking... easy to find out and no parts needed... yet. I recommend checking out the pistons before you install a new master cylinder first.

Pull the wheel, flip up the caliper... you should be able to tell.

Peace
Bruce in Philly (now Atlanta)