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

[Otter74]

You would have found that the wheel in question was a lot hotter than it should be.

Rebuilding a caliper isn’t expensive but PMB calipers are quite nice and I hate doing things like rebuilding calipers.

Originally Posted by famoroso

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)...



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



@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.

[stownsen914]

If the brakes got hot enough to fail due to a dragging caliper, you would have smelled it. At least once you pulled over, and maybe even from the cockpit while driving. Hard to describe the burning smell, but it's distinctive and very obvious.

[Locker537]

Since they are dragging, have you determined why?

Is the caliper piston(s) sticking?
Brake lines collapsing?
Something else?

[911pcars]

Brake pads will always rub slightly against the rotor without brake system pressure. A secondary function of the piston seals is to slightly retract the pads with zero system pressure.

In that there seems to be excessive rotational friction with the brake pads removed eliminates a stuck caliper piston and/or bloated brake lines. Instead, it indicates the rotational friction might be from another source (e.g. excessive wheel bearing preload, damaged wheel bearing, lack of lube, wheel weights rubbing on hub/caliper, etc.).

MHO,
Sherwood

[famoroso]

Originally Posted by stownsen914

If the brakes got hot enough to fail due to a dragging caliper, you would have smelled it. At least once you pulled over, and maybe even from the cockpit while driving. Hard to describe the burning smell, but it's distinctive and very obvious.

After hundreds of track events and race weekends over the past 30 years, I am familiar with the smell of hot brake pads doing their thing (I LOVE the smell of brake pads in the morning!). However, in this instance, I didn't smell anything, even from the cockpit while driving.

Originally Posted by Locker537

Since they are dragging, have you determined why?

Is the caliper piston(s) sticking?
Brake lines collapsing?
Something else?

I have not yet determined why. I had three days to get the car ready for paint, so apart from the two tests above (spinning the wheel with the car in the air and spinning the brake rotor with the pads out), I have yet to find the smoking gun. I trailered the car to the paint shop on 7/20. It'll be there for eight weeks, at which point it'll go to the upholstery shop for 3-4 weeks. After that, I'll trailer it home and work on the brakes. The two videos suggest it is sticking calipers / swelled lines and not "something else." Regardless, at a minimum, all four calipers will be sent to PMB or Tru6 for rebuilding and I'll replace all rubber lines.

Originally Posted by 911pcars

Brake pads will always rub slightly against the rotor without brake system pressure. A secondary function of the piston seals is to slightly retract the pads with zero system pressure.

In that there seems to be excessive rotational friction with the brake pads removed eliminates a stuck caliper piston and/or bloated brake lines. Instead, it indicates the rotational friction might be from another source (e.g. excessive wheel bearing preload, damaged wheel bearing, lack of lube, wheel weights rubbing on hub/caliper, etc.).

MHO,
Sherwood

Except that, per the second video above, there is no rotational friction with the brake pads removed.

Originally Posted by Otter74

You would have found that the wheel in question was a lot hotter than it should be.

Rebuilding a caliper isn’t expensive but PMB calipers are quite nice and I hate doing things like rebuilding calipers.

Calipers will go to PMB or Tru6 to be rebuilt. Most likely Tru6 as I need to have a number of other items coated / plated.

[Locker537]

Frank, the car is going to be a hoot when done...

[Jack Stands]

New paint and nicely refinished PMB Calipers would be a lovely combination. Brakes and steering are the two most important controls on your car.

[famoroso]

Originally Posted by Jack Stands

New paint and nicely refinished PMB Calipers would be a lovely combination. Brakes and steering are the two most important controls on your car.

Oh yeah!

Steering rack rebuild and Quaife Quick Rack rack & pinion install are on the to do list (along with replacing / refreshing the rest of the running gear) while the paint is curing.

[mikesarge]

Originally Posted by Ian Comerford

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'm going to 2nd this. It's either the master or the booster IMO.

[panzerfaust]

Master seals… my guess. I’m replacing mine on a M491

[fbarrett]

If you don't know how old the flex brake lines are, replace them. Old lines come apart inside, and the crud blocks return flow, so the pads keep rubbing the disk. As the car sat for a short while, fluid seeped back through the blockage, and the brakes felt normal again.

[sugarwood]

Originally Posted by Locker537

Since they are dragging, have you determined why?

Is the caliper piston(s) sticking?
Brake lines collapsing?
Something else?

What exactly happens when a caliper is seized?

[john walker's workshop]

Heat, heat, baby.

[pmax]

Test the brakes periodically when driving ... any car, not just our 40 year old classics ...

[kltarga72]

Originally Posted by john walker's workshop

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

Plus 1 with caliper issue.
Had a similar experience, when given time to cool brakes returned to normal. I always carry a laser temperature gun in glove box and was able to diagnose which wheel/caliper was faulty.

Car sat at shop for several weeks so could be either stuck caliper or master cylinder

[Shaun @ Tru6]

Hey Frank, scary stuff, I just rebuilt the right rear caliper on my M491, it had a slight drag. One of the pistons had a wear mark on it. New piston, new seals, it was good to go.

I'd be happy to do your set with the steering rack. Vents is looking great as a slick top!

Pics of a set I did a while ago. We ended up getting all new pistons as most were bad

[famoroso]

W00T!

Looks amazing!

I need to put a list together (calipers, steering rack, front control arms, etc.) for you so we can coordinate / define project scope.

[famoroso]

Originally Posted by fbarrett

If you don't know how old the flex brake lines are, replace them. Old lines come apart inside, and the crud blocks return flow, so the pads keep rubbing the disk. As the car sat for a short while, fluid seeped back through the blockage, and the brakes felt normal again.

I know how old they are... 33 years old. They're slated for replacement once the vehicle returns from paint and upholstery.

Originally Posted by sugarwood

What exactly happens when a caliper is seized?

Great thread, thanks for sharing.

Originally Posted by pmax

Test the brakes periodically when driving ... any car, not just our 40 year old classics ...

Amen to that brother pmax!

Originally Posted by kltarga72

Plus 1 with caliper issue.
Had a similar experience, when given time to cool brakes returned to normal. I always carry a laser temperature gun in glove box and was able to diagnose which wheel/caliper was faulty.

Car sat at shop for several weeks so could be either stuck caliper or master cylinder

Thanks for the tip. I'll add a mini IR temp gun to my toolbag.


Thanks for all the responses and input here. It takes a village. I'm probably going to go through and refresh the brake system on my 1987 too.