waddisme

If yes, then cool. If not, shouldn't the stuff in the hoses be bled out regularly? Especially in the fronts?

RX-Ben

The fluid does, the gunk does not. When your piston boots burn off, more dust gets in the fluid and this should be bled out. Same with clutch fluid (easiest w/remote bleeder).

1991Z07

I don't agree...

There isn't a "return" line from the caliper, so the system just compresses what is in the lines and modulates that pressure with the ABS pump.

To answer the OP's question: Yes, they should be bled. I do mine a few times a year. DOT3/4 brake fluid is very hydroscopic. It likes to absorb water from the air (even through rubber lines and seals).

Bleeding them regularly will keep moisture from corroding anything inside the system.

waddisme

I am embarrassed to say, but after 31 track days, I still have my boots on my front calipers.

So when I change my rotors out monthly for track days, should I just pump 1 pump to clear fluid from caliper and hose? Not sure why I would flush whole system.

RX-Ben

Clean and refill your clutch reservoir. Pump the clutch 10 times. Tell me what color your fluid is and tell me how it got that way w/o a return line.

for a system bleed, I typically do 2-3 bleeds per corner, after an initial couple pushes with the bleeder open to clear out the dirty stuff.

Sidney004

I don't agree...

The fluid circulates through the process of thermal diffusion. Very good advice on bleeding the lines regularly.

Aardwolf

It has been my experience that the fluid really doesn't circulate. I occasionally mix fluids and can clearly tell the color change when one has moved out of the lines.

Yes bleed a pump or two or till the gunk clears. I flush the whole system once per year.

1991Z07

Thermal has very little to do with it. Some, but not all. The dirt actually migrates from the motion of the fluid up/down the hoses and lines. You push down, then release and the dirt moves a little higher in the tube. Each time you do it it moves a little more, until finally the dirt is in the entire system. We did some of those types of experiments in school...very interesting when you can see it in clear Pyrex tubes.

Flushing the fluid regularly is key to keeping brakes in good operational order. If we could put DOT5 in the ABS system, there wouldn't be the worries of water. I know quite a few collectors with old Corvettes and their cars ALL have DOT5 silicone fluid in them. Keeps the systems from seizing up when they sit for long periods of time.

Short of that, DOT4 fluid is cheap compared to the costs associated with replacing calipers and ABS pumps.

A good reverse brake bleeder will pay for itself in a very short time. Practically guarantees there will be no air in the lines when you are finished. Some can be had for <$100.

P.S. We were able to completely "contaminate" a closed system with ZERO thermal influences in 50 cycles.

parkerracing

A google search brought up a bunch of junk but I remember about 10-15 years ago a company marketing a brake fluid cooler that recirculated fluid through a cooler/return lines and check valves. They were all the rage in NASCAR and Trans Am. Wonder what happened to them?

1991Z07

Now you're talking an entirely different animal...

Custom application for racing vs. the stock system on a Corvette.

Apples to oranges...and I do remember something about that as well.

Wouldn't surprise me if they found a sketch from Smokey Yunick and used it. He was a "real" outside of the box kinda guy.

Things like "reverse flow cooling"...

Everett Ogilvie

The fluid does not circulate and is in fact quite static if it does not boil. It gets pressurized and transmits force but there is very little natural "mixing" of the fluid. And particles do not move from one end of the fluid to the other due to the force. The force is exhibited at the opposite end of the fluid from where it is applied due to the fluids incompressibility, i.e it behaves virtually as a solid - apply a force on one end and the same force appears at the other end (even traditional frictional losses do not apply because those are for fluid flow and there is no flow in this system).

If you get the brakes hot enough so that the fluid boils because it has a relatively low wet boiling point (cheap brake fluid) there is mixing in the part of the system where there is boiling, still likely not the entire system.

dvandentop

great explanation Everett

Everett Ogilvie

This is another one of those interesting discussions and like a true nerd I thought about it most of the night, trying to recall my fluid mechanics and mass transport.

I definitely agree with 1991Z07 who posted above about the action of the fluid eventually distributing any particulates, and given enough time they should become fairly equally distributed throughout the fluid.

The "flow" is not quite like classical internal flow (a fluid moving through a pipe) but the governing physics are still the same. The velocity is zero at the wall (bounday layer) and has a gradient from zero to the max velocity U in the center of the pipe (the brake line). That velocity is quite small, again, because there is not true flow/circulation, but the fluid column does move a few thousandths as the pistons extend within the caliper, and the fluid column then moves the opposite direction as the pistons retract.

One more thought - because there is a velocity gradient within the fluid (zero at the wall, some tiny velocity in the center) there is shear within the fluid (as there is in all internal flows, and external flows for that matter) - it is this shear which drives the particulate transport, and now that I have thought about it a bit more, this is indeed mixing of the fluid, although on a pretty small scale and still not what I would call circulation.

waddisme

Adding quick bleed to maintenance check for sure. I am with RX-Ben though, how come that doesn't work for the clutch? On Ranger's website, he showed how clutch dust was getting into the system to discolor fluid in resevoir.

waddisme

OK, should have waited one more minute and that would have answered my question. Makes sense now.

Sidney004

We all agree that there is no flow of particulates(mostly elastomers I would guess) through the system. We also agree that the fluid is static with the exception of the small piston movement.
However, the laws of diffusion still apply to a brake system. There is always a natural mixing of fluids that are miscible, in this case brake fluid and water. It is decreased by the diameter and length of the brake lines but driven by the thermal gradient and overall temperature, the time required for complete mixing(shall we call it circulation) is certainly a real number.
One of the primary reasons for bleeding is to ensure that the chemical properties of the fluid(maintain a high boiling point) at the business end(the caliper) are optimal and not degraded by a high water content. Our brake systems aren't sealed and I would think that the primary entry point for moisture is the master cylinder reservoir. If you were able to measure the properties of the fluid(water content) would you expect uniformity or a measurable difference between the master cylinder reservoir and the bleeders of the calipers? As a practical observation, I use ATE (blue and yellow dyed) fluid in our Mercedes and on a complete flush, the color change is distinctly noticable; otherwise its always light blue. I believe there is complete mixing of all miscible fluids.
By the way, it is an interesting discussion and I too am attempting to recall my fluid mechanics.

varkwso

My pupils dilate and my breathing gets shallow when we talk like this.

Diffusion will always happen in any fluid. Equilibrium is the preferred natural state so everything moves to that state. Circulation is probably a strong term but there is definitely a mixture gradient in our brake system. The practical application is bleed often. Particulate helps you track movement easier BTW.

whatcop?

Moral of the story bleed the very end of the fluid at the caliper before race/HPDE and after if you really want to keep it fresh. Also use a pressure bleeder.


Sorry meant to say bleed

sothpaw2

Right--I would call this more of a bleed at all 4 calipers. Previously, I flushed the whole system (1 Litre) before every event. I think that's excessive. I'm going to try just replacing the stuff in the reservoir and then bleeding the calipers quite a bit. I think this is all that's needed.

RX-Ben

That is overkill (regular full flush). I'll go a few events between bleeds. When I do bleed, it is for no particular reason. Full flush every 3mos or so.