DOT 4, DOT 5.1, and DOT 5 Silicone

Q: My bike’s brakes are currently filled with DOT 4. Is it true that I will get better performance if I switch to Bel-Ray Silicone DOT 5 Brake Fluid?

A: No, Years ago, many road race teams used DOT 5 silicone brake fluid to get higher boiling points than the DOT 3 & 4 brake fluids of that era. DOT 5 silicone is not hygroscopic, meaning it won’t absorb water from the air, so it retains its high boiling point over time. On the other hand DOT 3, 4 and 5.1 brake fluids are glycol based and do absorb water from the air, which then lowers the boiling point. That is why you will see wet and dry boiling points, with the dry boiling points always being higher. Glycol based brake fluids may start out with a fairly high dry boiling point but as they absorb moisture the boiling point decreases. So every time your brakes get really hot and then cool, condensation occurs and moisture is absorbed. That is why it is important to replace your brake fluid regularly. It may seem obvious that DOT 5 silicone is the best choice, but not so. Today most road race teams use DOT 4 or DOT 5.1 glycol based brake fluids because the dry and wet boiling points today are very high and in some cases higher than DOT 5 silicone. The number one reason not to use DOT 5 silicone is a slightly mushy lever or pedal feeling as compared to glycol based brake fluids. This is the nature of the product. Street riders may not feel the difference, but the race teams prefer the precise feeling of glycol. The only motorcycles that I know of rolling off the production line with DOT 5 silicone are Harley-Davidson® and other American made motorcycles; and that is mainly where we sell our Bel-Ray Silicone DOT 5 Brake Fluid. I recommend you stay with the DOT 4 fluid for performance, not to mention the enormous job it is to switch your system over from glycol to silicone. If that job is not done properly, you run the risk of coagulation if remnants of DOT 4 are mixed with DOT 5 silicone. I hope that’s not too much for you at once. Maybe you should read half, take a play break, then read the rest.

DOT 5 Silicone versus DOT 5.1

Q: The master cylinder cover of my bike says fill with DOT 5 brake fluid. Can I use your Bel-Ray Silicone DOT 5 Brake Fluid?

A: I’ll bet your bike is of European descent! In that case do not use DOT 5 silicone in your brakes. Use DOT 5.1 brake fluid. There is much confusion around this dilemma and making an error could be serious. DOT 5 silicone brake fluid is generally not recognized in Europe, so they call DOT 5.1 DOT 5 for short. In America, the brake fluids’ names are DOT 3, DOT 4, DOT 5 and DOT 5.1. All are glycol and compatible with each other except for DOT 5 , which is, obviously, silicone based. In Europe beware, they do not recognize DOT 5 silicone and, therefore, call DOT 5.1, which is a glycol product, DOT 5. This can cause great confusion and obvious problems. DOT 5.1 got its start during the introduction of anti-lock or ABS brakes, where a light viscosity brake fluid was used to assist the modulation of the anti-lock mechanism. Suddenly DOT 5.1 is in vogue today and many are buying it under the guise that it must be better than DOT 4 because it is the next sequential number. Not so! To get the best performance brake fluid, check the wet and dry boiling points. In many cases, depending on the manufacturer, DOT 4 has higher boiling points than DOT 5.1. Today, DOT 5 silicone brake fluids are mainly used in the Harley-Davidson® motorcycles since this is what they come with from the factory. The advantage is that if any fluid spills, it won’t damage the custom paint jobs. Do your community a favor and pass this info on to another European bike owner or, better yet, quiz your dealer to see if he is "in-the-know".

DOT 4 VS. DOT 5 Silicone Coagulation

Q: I bought a 1987 Harley-Davidson Sportster® and I recently removed the front brake master cylinder cover to find a bunch of thick gunk inside. What happened and what should I do?

A: If you own a Harley-Davidson®, think purple! Your H-D® motorcycle came stock from the factory with DOT 5 silicone brake fluid and part of the regulated requirements for DOT 5 silicone is that it be purple colored so as to stand apart from the ever so popular DOT 3, 4 or 5.1 brake fluids which are glycol not silicone. Apparently, your hog’s previous owner was more concerned with his tattoos turning green than with his brake fluid being purple and most likely added some DOT 3 or DOT 4 brake fluid to the system. This is a big no-no, bad boy! What happens when you introduce DOT 3, DOT 4 or DOT 5.1, all glycol based fluids, to DOT 5 silicone, which is obviously silicone based, is coagulation! The fluid will clot and, as you so eloquently described, form “thick gunk”. My advice to you is to consult a qualified technician. After all these are your brakes we are talking about! Have him fully flush the brake system and replace all the rubber parts with new ones and then go back to DOT 5 silicone brake fluid. If you do not re-build the master cylinders and calipers or replace all the rubber parts, there will most likely be remnants of the glycol in the rubber which will cause the problem again. To avoid "thick gunk" in your shorts due to grabbing the brakes and not impeding forward velocity, I recommend you heed my advice with the utmost seriousness and expedience.

Original article here
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