CVO Technical > Drive Train
Stop and go traffic 300* oil temp no clutch!
Para Bellum:
--- Quote from: Rimjam on December 05, 2019, 11:02:54 AM ---Excuse me for being dumb but I don't understand the mechanics of what's going on here. The brake fluid is apparently boiling from exhaust heat releasing oxygen (air) from the fluid & creating 'bubbles' in the line? Where's the fluid going that's displaced by the bubbles, into the master cylinder? I do understand that air bubbles then compress when lever pressure is applied & cause the system to fail but how is the system restored when the fluid cools down? Is the oxygen (air) reabsorbed into the fluid?
--- End quote ---
The answer to your questions is "Sorta." :)
Anything in the hydraulic system that is already a gas (like air) or that can turn to gas (i.e., a fluid that boils) can cause loss of braking, ranging from a spongy pedal/lever to flooring the brake pedal/lever (total failure).
The most common things in a hydraulic system that can turn to gas are air, water, and hydraulic fluid (HF)...yes, even HF can boil, although its boiling point temperature is much higher than water. The first HF was water, but soon it was changed to oil to deal with the boiling problem.
As you noted, the bubbles compress when you squeeze the lever, but the slave cylinder doesn't move the brakes (or clutch) until all of the gas has been squeezed back into liquid. If the master cylinder reaches full stroke before all the gas is turned to liquid...hello, foot to the floorboard.
BTW, when the HF exceeds the breakdown temperature, the oil molecules split into smaller molecules. The problem then: the smaller molecules boil at a lower temp than the original full-size molecules, and the fluid loses viscosity (thickness). Then the fluid can leak past the pistons in the cylinders, with the same loss of function as above. This is why I use synthetic HF; just like synthetic engine oil and tranny fluid, it stands up to high temperatures better than dinosaur juice. It does degrade over time, though, so it needs to be changed out. Since your HF already has been boiled, it's definitely time to replace it.
Rimjam:
But neither DOT 3, 4, or 5 are synthetic, are they? Who makes synthetic & what's its trade name?
Para Bellum:
--- Quote from: Rimjam on December 05, 2019, 07:04:01 PM ---But neither DOT 3, 4, or 5 are synthetic, are they? Who makes synthetic & what's its trade name?
--- End quote ---
Many companies make syn HF, such as Lucas, Shell, Castrol, AMSoil, Mobil, etc. You can get it at any auto parts store like O'Reilly's, AutoZone, Pep Boys. Just look for the word "Synthetic" on the label.
Use DOT 4 for the bike.
Rimjam:
All of my oldie-goldie stuff - '80 Sturgis, '94 RK, '00 RG - came with DOT 5. Any reason I should change to DOT 4? Is there such a thing as synthetic DOT 5?
deldago:
DOT 5 brake fluid is silicone based. DOT 3 (standard) and DOT 4 (heavy-duty) are glycol-based. It can be distinguished from conventional brake fluids by its purple color (which comes from a dye).
Silicone does not absorb moisture. DOT 5 brake fluid does not become contaminated with moisture over time as conventional DOT 3 and 4 brake fluids do. Silicone is also chemically inert, nontoxic and won't damage paint like conventional brake fluid. It also has a higher boiling point.
Because of this, it is often marketed as a premium "lifetime" brake fluid. It is often used to preserve brake systems in antique vehicles and those that sit for long periods of time between use.
DOT 5 silicone brake fluid is also very expensive (costing four to five times as much as ordinary brake fluid), and it won't mix with glyco- based brake fluid (creating concern over sludging if all old fluid isn't removed when a system is refilled with silicone).
Silicone also has slightly different physical properties and compressibility, making it unsuitable for ABS systems calibrated to work with DOT 3 or 4 brake fluid.
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