Name that brake failure mode! Pedal goes to the floor w/o warning. Stranded.
 

Ok, here's the situation.

1989 911 with 135,913 miles. Sitting in a parking lot in Santa Fe Springs for the last 5+ hours, waiting for a third flatbed.

Dropped the car off at Perfect Lines in North Hollywood for a sunroof delete on June 7. Picked it up today, July 16, for the 117 mile drive home to Carlsbad / Encinitas.

http://forums.pelicanparts.com/uploa...1626481477.jpg

http://forums.pelicanparts.com/uploa...1626481477.jpg

http://forums.pelicanparts.com/uploa...1626481477.jpg

47 miles into the drive, while doing 65 mph on I-5 Southbound, I applied the brakes and the pedal went to the floor, without the car slowing at all. I applied the pedal a few more times with the same result. I then pumped the brake pedal, no improvement.

The pedal felt exactly like it would were a brake bleeder screw open... straight to the floor with very little effort and ZERO braking action. The brake system warning light in the dash did not illuminate.

I was able to use the handbrake to slow the car down, move into the right lane and take the next exit. I pulled into a commercial complex parking lot, in Santa Fe Springs, and shut the vehicle off.

I popped the frunk, expecting to see an empty brake fluid reservoir.

http://forums.pelicanparts.com/uploa...1626481348.jpg

It was at it's normal max level. I looked around the master cylinder and the brake calipers for leaks. None found.

I called my Porsche factory trained master mechanic uncle and described the problem to him. He said while he has never been able to figure out this particular failure mode, it did happen to his customers back in the day. They always just replaced the master cylinders.

As I was speaking to my uncle, about five minutes after pulling off the freeway and shutting the car down, I began to remove the driver's side floorboard (to inspect the pedal box) and brushed the brake pedal with my hand. It felt normal. I pushed the pedal with my foot, it felt firm / normal. Started the car and drive it around the parking lot a few times and it feels normal.

Not wanting to risk anything, I called my insurance company's, American Modern, roadside assistance line, only to have them claim the car isn't listed on my policy (even though I have a proof of insurance card showing it).

So, I call AAA at 2:30p to order a flatbed. It was supposed to arrive before 4p. I get a call from Hadley Towing at 5p claiming the truck that was slated to arrive broke down. So, I call AAA back for my third attempt at roadside assistance and here I sit, waiting for a response, posting on Pelican from my handheld.

So, I thought I'd post a "Name that brake failure mode!" thread here and get the collective group's thoughts. Any ideas?

I'm having a hard time reconciling M491 / turbo brakes that were working perfectly fine moments before, untaxed (it isn't like I was on the track), completely failing for at least 2-3 miles and then all pedal feel coming back less than 10 minutes after shutting down? No brake warning light, no easily discernable leaks. Both circuits failing concurrently? And now the pedal feels as firm as ever in less than ten minutes of sitting. I've started the car a few times and driven it around the parking lot and the brakes feel... Normal, which is to say firm / great.

So, whaddaya got? Any ideas, or is this the beginning of me losing my mind?

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I think id bleed the brakes at the back where the line gets cooked by the cat. just a guess

Thank you.

B&B headers FWIW.

Seen it with contaminated brake fluid where the customer adds oil or power steering fluid. Causes seal failure inside M/C. I’d start with the M/C if visible leaks are absent....

Thank you. Hadn't thought of someone introducing contaminants.

my brother made that mistake at work once. The brake fluid bottle looked exactly the same as the power steering bottle. It happens

Five hours and fifteen minutes later and I'm finally on the back of a flatbed thanks to Victor at Hadley Towing for accepting a trickier job (low Porsche) that I assume other drivers just passed on the past four hours

Full brake system refurbish / rebuild just jumped to the top of the to do list.

http://forums.pelicanparts.com/uploa...1626488951.jpg

http://forums.pelicanparts.com/uploa...1626488951.jpg

Caliper(s) drags, fluid boils and becomes vapor, vapor compresses, pedal drops. Fluid cools and brakes are back.

Thanks! That'll be the first thing I check when she's up on the lift tomorrow. This is a totally plausible explanation!

I'll also check brake fluid moisture content.

Not really possible in a brake booster 911 I guess but in my 914 I did have the fuel tank pinch the brake line between the reservoir and the master once the tank settled. Replaced the master twice before figuring out that issue on why the brakes would feel fine during a bleed and then go to nothing after a short drive.

Glad you were able to get to off the freeway without incident.

Scary. Glad you got off the interstate ok. JWs hypothesis makes sense. Let us know what you find.

Given your explanation of the circumstances I think you will find the master cylinder faulty. There are no visible failure modes but the pedal going to the floor without warning is classic for this

I’d think a rotor would be blued up good if a caliper got hot enough to boil fluid.

It happened to me once after 4 intense laps around the track w/ lots of threshold braking. Pedal to the floor but slowly regained the brakes after deciding it was cool-down lap time. Changed brake fluid & all was golden again.

John Walker’s suggestion is the first thing I’d check for, even with interstate driving. Did you check the temperatures of the wheels when you pulled over? I had a sticking caliper on my car that gave me exactly the same failure mode and subsequent behavior. Rebuilt the offending caliper the next morning.

MC in trunk should provide easy access. Loosen a brake line fitting on the master cylinder, have a helper apply the brake pedal and see if fluid is forced out. Then repeat on the other brake line. If no fluid pressure, suspect a bad piston seal. Not sure if MC rebuild kits are still available. If not, try a rebuilt unit.

Sherwood

- should be interesting to see the forensic exam on this one...

I forget when the 'safety split' of brake lines happened, but a complete and quick failure on initial application means it has to be in the m/c somewhere.

Boiling fluid should not happen on initial appl. tho you should flush/refill your fluid...

Did that sunroof deleters do any other service on your car?

++ for JW, You did not say, but have the calipers ever been touched, or are they original?(suspect)

Yep. Dragging caliper is my bet too.
I don't mess around with brakes. They gotta work. If they're OEM and untouched they're 32 years old.
If it were my car I'd pull all four send to PMB, then replace all the rubber (braided) lines, probably replace the MC while I was at it, fresh rotors, full flush, thorough inspection of the hard lines. After that you'll be able step on the middle pedal with confidence.

As per usual, it appears that Herr Walker was correct. Send that man a cigar! (I will be)

Here is a vid of the front wheels, spinning (or at least trying to)...

<iframe width="560" height="315" src="https://www.youtube.com/embed/I4zMvZpjFuQ" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

Here are the front rotors spinning with the brake pads removed...

<iframe width="560" height="315" src="https://www.youtube.com/embed/ZSyZ-_hb9K4" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

@Otter74 I did not check wheel temps when I pulled the car over. I will add that yesterday was the highest ambient temps (85+ degrees) that I have driven the vehicle in since I acquired it this past February.

@Thor66 The guys at Perfect Lines only worked on the sunroof delete.

@Eastbay Calipers appear to be original.

@Charles Freeborn A comprehensive brake system overhaul is on the docket. Including, as a result of the above vids, rebuilt calipers at all four corners.