RE: Mighty Vac & Brake Bleeding

[dirtrider]

8 hours ago, PadG said:

Actually, the boiling point that one see quoted are usually the boiling point at atmospheric pressure.  OTOH, the pressure that the fluid in your brake circuit "sees" is the much higher pressure of the applied pressure applied by you when braking, which will correspond to much higher boiling point of the fluid.  So, elevation doesn't really have any effect on it at all, since it doesn't affect the internal brake circuit pressure!

 

Morning PadG

 

You might want to think on this one a bit more.

 

That is unless you or someone that you know rides continually with the brakes applied & pressurized.

 

If you brake hard & long enough to raise the brake fluid temperature to above  it's boiling point (at local atmospheric pressure) then, while under braking pressure,  that might 'momentarily' prevent brake fluid boil, BUT,  the second that you quit braking & the brake system pressure drops to local atmospheric you will have an instant fluid boil. 

 

If you don't believe this then borrow your wife's pressure cooker, then add water & bring the water temp up to 220°f with the pressure cover on. 

 

Now simply remove the cover, tell us what happens? 

 

 

 

 

[realshelby]

1 hour ago, PadG said:

Actually, the boiling point that one see quoted are usually the boiling point at atmospheric pressure.  OTOH, the pressure that the fluid in your brake circuit "sees" is the much higher pressure of the applied pressure applied by you when braking, which will correspond to much higher boiling point of the fluid.  So, elevation doesn't really have any effect on it at all, since it doesn't affect the internal brake circuit pressure!

Well, I beg to differ. Yes, while under pressure boiling point changes. But what happens when there is no pressure on the pistons in the caliper? That means the system is again at atmospheric pressure and the residual heat from the braking package can and does boil the fluid sooner than it would at sea level. Once the "bubbles" are there, simply building pressure by applying the brake won't overcome them. 

[Pappy35]

2 hours ago, PadG said:

Actually, the boiling point that one see quoted are usually the boiling point at atmospheric pressure.  OTOH, the pressure that the fluid in your brake circuit "sees" is the much higher pressure of the applied pressure applied by you when braking, which will correspond to much higher boiling point of the fluid.  So, elevation doesn't really have any effect on it at all, since it doesn't affect the internal brake circuit pressure!

 

True but for the instant after the pressure is released. Just after coming off the level the brake piston temperature will be at it peak and since the pressure is at that moment back to 1 atm the boiling we're talking about can occur. Of course, as has been previously mentioned, this would only be true in extreme conditions (very neglected oil with high water content combined with extreme braking as in racing).

 

Edit: As others have pointed out, extreme conditions are rare but don't just happen in racing conditions. 

[LAF]

Folks CHANGE YOUR BRAKE FLUID!  Simple job with a speed bleeder, with a mighty vac, or pump, pump, close.

 

Use a turkey baster to empty the reservoirs, a lint free cloth to wipe them out, fill them and do the flush however you want.

 

I do it as I said because it gets yellow and I live in a tropical rain forest here in PA in the summer. 

 

I also do it as I read of the "wizzy" brake ABS modules of the past that are dead and need rebuilt or bypassed.  I always wonder if guys would have flushed the brakes more, would those have had better service life?

 

A lot of technical stuff here and stuff I never new especially using voltage on brake fluid.  It makes sense as I know they use current to measure all kinds of fluids.  The way it attracts water molecules is pretty cool to learn also.

 

Everyone has their own way of taking care of their bikes.

[PadG]

22 hours ago, dirtrider said:

 

Morning PadG

 

You might want to think on this one a bit more.

 

That is unless you or someone that you know rides continually with the brakes applied & pressurized.

 

If you brake hard & long enough to raise the brake fluid temperature to above  it's boiling point (at local atmospheric pressure) then, while under braking pressure,  that might 'momentarily' prevent brake fluid boil, BUT,  the second that you quit braking & the brake system pressure drops to local atmospheric you will have an instant fluid boil. 

 

If you don't believe this then borrow your wife's pressure cooker, then add water & bring the water temp up to 220°f with the pressure cover on. 

 

Now simply remove the cover, tell us what happens? 

 

 

 

 

Hi DR,

 

Good point!  You are right!!!

[PadG]

22 hours ago, realshelby said:

Well, I beg to differ. Yes, while under pressure boiling point changes. But what happens when there is no pressure on the pistons in the caliper? That means the system is again at atmospheric pressure and the residual heat from the braking package can and does boil the fluid sooner than it would at sea level. Once the "bubbles" are there, simply building pressure by applying the brake won't overcome them. 

Yep, you and DR are correct!  OTOH, unless you apply the brake again right after that, the bubbles really doesn't matter, BUT chances are high that the brake will be applied again right away, since most of us usually "pulse" brake when braking hard, as a habit to help cool the rotor.