Strange Brakes

[MickG]

I don't think we were all wrong
By what you describe (the boiling brake fluid) you are in my opinion experiencing vapour lock. The fluid boils producing air bubbles that you can't compress; this happens more often with old brake fluid due to it's hygroscopic nature and water having a much lower boiling point than brake fluid.
Assuming clean fresh Dot 4 fluid the boiling point is 230 degrees C, old stuff with some moisture added 155 degrees C
Brake fade is due to overheating brake material and is something different to what you have been experiencing/describing.
I'm glad you have found the cause of the problem as when it happens it make you s!&% bricks as anyone who has experianced this will tell.
PS I do know that you can compress a gas but not with the pressures involved in this case.

[Foxie]

PS I do know that you can compress a gas but not with the pressures involved in this case.

IMHO Some dodgy physics here.

Gas are always compressible, whether air, water vapour, or boiling brake fluid. Boyle's Law.

Liquids are not compressible. That's how they transmit force directly, hence "hydraulic system"

How come i didn't hear about this run in Galway ?

[simonknee]

The problem is obviously brake fade from boiling fluid. The root cause is the seized caliper piston. I was planning to take off the wheels after the run anyway to get them stripped and painted while I await the new tyres I ordered so I guess I'll be doing a caliper (or two) rebuild while it's up on blocks!!

I think I'll pour some cold water on this theory (if not your brakes). The issue is primarily with your overheating front left but brake fade is not the explanation. Brake fade doesn't mean your pedal goes to the floor, it means you don't get as much stopping action as you expect due to the friction materials not working as well when hot. Check "Causes of Brake Fade" on http://en.wikipedia.org/wiki/Brake_fade. Spongy yes but not full travel!

Unless it is _completely_ explainable from what is happening at the left front I would at least check out the master cylinder too. Perhaps it has a problem with seals that the hot fluid is exacerbating. Hot brakes/fluid can happen in normal driving and it the last point that you want another weakness to show itself.

Simon

[rgh0]

lets have a go at getting the science right

A dragging pad due to a sticking piston overheats the brake fluid in the brake caliper.
The brake fluid boils and generates vapor. Brake fluid is displaced back into the reservoir by the vapor.
You press the pedal and the vapor is compressed by the fluid pressure in the brake system
The pedal goes all the way to the floor but the pressure in the system is still very low as the vapor is compressible
The low pressure in the system results in very little braking effort.

Later after the system cooling the vapor condenses and the fuid returns from the reservoir to replace it.
The brakes are normal again when cold

cheers
Rohan

[MickG]

Sean Murrey Wrote

IMHO Some dodgy physics here.

Please explain the dodgey physics Sean?
The PS stated that I know you can compress a gas, and I do know that you cannot compress a liquid. But in my opinion the amount of pressure required to compress a gas sufficiently to form a liquid in this case would be insufficient.
I think Rohan has explained what I was try too say far better then me.
Regards MickG

[Galwaylotus]

Rohan, as usual, far more eloquent than I but that's exactly what I was trying to say.

[Galwaylotus]

Brake fade doesn't mean your pedal goes to the floor,

Although I accept the terminology difference, Fred Puhn's Brake Handbook states, "Brake fade is loss of braking due to overheating. It can cause longer pedal travel as the brakes get hotter - maybe to the point where the pedal goes to the floor." (italics mine)

Vapour lock is more commonly used for fuel issues but that is the issue I described above that relates to my brake temporary failure.

[simonknee]

Blimey I suppose I am only thinking of the kind of fade you get in spirited road driving.

This kind is due to a fault mode - or can it happen when racing???

EDIT: of course it basically happens when your fluid reaches boiling point whatever you are doing at the time. So what I am really asking is this ever likely under normal conditions when heavy braking? Or put another way has the "pedal to the floor" scenario ever occurred to anybody and it not been due to a fault mode. I'm not arguing here (I was wrong in my previous post) but curious as to the limits of our brakes.



SImon

[Jeff@Jae]

Old brake flex hoses can also cause a dragging caliper. During brake use they can swell and on release they cause a residual pressure back to the caliper when they contract. Inexpensive in either the original rubber or in stainless steel braided teflon types.

[Elanman99]

Brake fade doesn't mean your pedal goes to the floor,

Although I accept the terminology difference, Fred Puhn's Brake Handbook states, "Brake fade is loss of braking due to overheating. It can cause longer pedal travel as the brakes get hotter - maybe to the point where the pedal goes to the floor." (italics mine)

Vapour lock is more commonly used for fuel issues but that is the issue I described above that relates to my brake temporary failure.

I do not have the book you refer to but with or without the italics the part of the text quoted does is not very helpful. He does not say what is overheating so I cannot see how it adds anything to the discussion.

I wonder whether he is referring to drum brakes though? The drums themselves increase in diameter when hot causing the pedal to get ever nearer the floor, I had it once on a car with front disks and drum rear brakes, I can tell you it was quite stressful as I was descending a mountain pass into Italy!

Ian

[simonknee]

Hmm, but Rohan's explanation convinces me Ian.
Up until then I could not see what had changed enough to cause a FTTF (Foot To The Floor) scenario.

Seems unlikely the couple of mm your drums expand by would cause FTTF.
Perhaps this too was the vapor produced by boiling brake fluid.

On a separate note about gas compression.
There is obviously enough pressure available in a car braking system to compress any gas (or vapor) present.
Otherwise why would we bother bleeding out the pesky air bubbles!
However why do the brakes firm up if there is vapor? Is this what you meant, MickG?

Simon

[Galwaylotus]

Another way to look at it is that the vapour or gas in the system (whether air or brake fluid or water vapour from boiling the fluid) acts like a soft spring vs the very stiff spring effect of the liquid phase of the brake fluid (yes, even liquid is compressible - just much less than a gas. That's why racers don't use silicone brake fluid!!). You'll still get compression but against a "soft spring" there isn't much force reacting against the pad/disc interface.

[dpo#4]

Howdy, folks,

I'll take a shot at this, too. "Brake fade" may refer to: (1) The loss of proportionality between increased pedal pressure and the hoped for increase in braking result, and (2) brake pedal to the floor, sometimes remedied by quick reapplication(s). While I'd prefer usage (1), I've seen (2) as well.
Some causes are: (1a) Decreasing coefficient of friction as pads heat up. Original equipment pads are formulated to stop well on the first try, but not so much after the tenth. Metallics are great when hot , but when they first came out we had a client roll thru the stop sign near his home on the first cold stop the morning after a brake job. (1b) Gas generated from vaporised binder (glue) floating the pad or shoe away from the rotor or drum. Also, wet pads. Think air hockey or hovercraft. Drill rotors, groove pads, use sintered pads (1c) Drums bellmouthing as they heat up, resulting in loss of contact area. These are high pedal, high effort conditions.
PTTF causes are: (2a) Pad knockback. Pumping works. Pedal may be low but feels firm once freeplay is taken up. (2b) Fluid leaks , internal or external. Pumping may work, at least at first. Pedal sinks under prolonged pressure (2c) Air or boiled fluid in the system. Pumping doesn't help. Cooling works for boiled fluid. We all know what air feels like. The nearly "solid" column of fluid trapped between our master cylinder seals and caliper seals now has a lower rate "spring" of air inserted that uses up our pedal travel. Pressure is being applied to the pads, but given enough air we hit the floor or the end of the master before it's enough to lock up the wheels. The usual description is "spongy".
In this instance, Galwaylotus has almost certainly found the cause in a dragging caliper. I say "almost" because I have lots of experience in auto repair, and I once read that "experience" is another word for "mistakes". Lots of experience over here. - Ray

[simonknee]

So re-reading all the info in the thread the bit the still doesn't fit is: if the vapor of boiling fluid is causing a FTTF then why do a couple of pumps firm things up?

This is all really interesting since knowing what happens in these scenarios, and how to react, is potentially life saving.

[MickG]

Simonknee wrote

However why do the brakes firm up if there is vapor? Is this what you meant, MickG?

This has got all to complicated for me and to be honest I forget what I did mean

Perhaps I am best left-understanding things as I know them rather than getting too technically involved.
If it were me, I would give the braking system a good overhaul and satisfy myself that all was OK.
It would appear that many different scenarios could exist; and any one could be the cause of Galwaylotus's problem.
It doesn't matter who is right or wrong so long as the problem is fixed.
Regards MickG