Strange Brakes

[Henry VIIII]

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.

My thought on that are that the vapour whether it be boiled fluid or water vapour driven from the fluid, will be in the brake cylinder the nearest point to the heat source; when you release the brake pedal the expansion of the vapour is not as quick as the normal retraction of the brake piston, so the fluid in the pipe does not move back to the master cylinder as quickly as normal, this causes the master cylinder to draw in some small quantity of fluid from the reservoir.

Repeat pedal release & re-press a couple of times and you have enough fluid in the system to compress the vapour to small dimensions and you gain a somewhat spongy brake performance.

[Foxie]

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

Agree completely.

Brake fluid, like all fluids, is for all practicible purposes incompressible. Gas is not. Typical brake system operating pressure is typically around 1000 psi, or 70 bar. Any vapour (or air) must first be compressed to 1/70th of its volume at ambient pressure to raise its pressure to brake system operating pressure. Typical master cylinder displacement is in the range 0.5 to 1 cubic inches. If there is only a small amount of vapour (or air) repeated pedal strokes may acheive operating pressure, but if there is significant vapour that becomes impossible.

Also, bulging flexible hoses only result in extra and springy pedal displacement. They do not cause delayed piston retraction.

(Runs for cover )

[simonknee]

Repeat pedal release & re-press a couple of times and you have enough fluid in the system to compress the vapour to small dimensions and you gain a somewhat spongy brake performance.

Yeah I buy this - scary

[Galwaylotus]

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.

Perhaps I wasn't clear. The first time the issue occurred I just slowed gradually and drove slowly for a bit and when I tried again, I had brakes. The couple of pumps were after a bit of cooling. I'm sure if I kept using them hard I'd lose them again but what likely happened is that there was sufficient cooling for the fluid gas phase to condense back into liquid. On Saturday's run, the last leg was a 30-mile non-stop run home and the brake and hub had gotten so hot that even after going very gently for the last three or four miles the knock-off nut was FAR too hot to touch! I still had virtually no pedal at that point; however, when I went out to the car about an hour later I had full pedal, i.e. all the gas had condensed into liquid. The hub was still hot but not nearly as hot as when I first got home. I pulled all four wheels this evening and will rebuild the calipers and suspension bushings while I wait for my new tyres to arrive and my wheels to be painted.

[simonknee]

Ah, yes that does make more sense now.

OT: What's the shield emblem on the wing?

[Galwaylotus]

Ah, yes that does make more sense now.

OT: What's the shield emblem on the wing?

The P.O. was a retired Scots mechanic so it's the Scotland badge!

[MickG]

And change the brake fluid

[Galwaylotus]

But the boiling would have evaporated any water vapour!

It goes without saying that the brake fluid will be replaced. It is two years old after all!

[MickG]

Good for you
Hope it all goes well.
Regards MickG

[Galwaylotus]

Ok, folks. Thanks for all the input. Now that I know where I'm going I have one more question. Can anyone tell me where I can get stainless steel caliper pistons? Canley Classics sells them for the front calipers (Triumph Spitfire) but I haven't seen any to fit the rears. As the root cause of my recent brake problems seems to have been a seized piston, I suspect it may be corroded so rebuilding the calipers is probably a good idea. If I can get ss pistons I won't have to worry about seized pistons from corrosion in the future.

[fatboyoz]

Hi Galwaylotus,
Try here: http://etypeparts.myshopify.com/collect ... ts/19-1209
and here: http://www.rdent.com/index.html
and here: elan-archive-f16/stainless-brake-caliper-pistons-t12583.html

Regards,
Colin.

Ok, folks. Thanks for all the input. Now that I know where I'm going I have one more question. Can anyone tell me where I can get stainless steel caliper pistons? Canley Classics sells them for the front calipers (Triumph Spitfire) but I haven't seen any to fit the rears. As the root cause of my recent brake problems seems to have been a seized piston, I suspect it may be corroded so rebuilding the calipers is probably a good idea. If I can get ss pistons I won't have to worry about seized pistons from corrosion in the future.

[Galwaylotus]

Just an update. I've pulled both front calipers and found that the pads had to be driven out with a hammer and drift. When the new ones go in after the caliper rebuild, I'll have to ensure that they can slide freely in the calipers. My left front pads, as well as being worn were, not to put too fine a point on it, - fried!!
Here are a few photos showing the pads from the left and right front calipers. They do not have many miles on them and they were installed at the same time. I haven't pulled the pistons yet but will be replacing all of them as a precaution along with new seals of course.

[Matt Elan]

If your pads had to be driven out with a hammer then IMHO that is probably the problem. If the pads are not free in the caliper than they will stick - and if they stick when they are in contact with the disk then that coult well be causing you the heating issue....
BUT after that has happened I'd refurb the calipers anyway as the heat won't have been kind to the seals.