synthetic brake fluid

[jsalmon]

My '03 R1150RT is ready for new brake fluid. The manual says DOT 4, is synthetic OK? Why must the ABS servos be bled? Won't all the old fluid flush out from the front and rear reservoirs?

[Andy VH]

If the manual calls for DOT 3 or DOT 4 fluid that is the only choice. If the synthetic brake fluid is also rated as DOT 3 or DOT 4 it should be just fine as the fluid would still have to compatible with the brake system seals. Yes, the ABS system bleeders have to be serviced also. The bends in the system tubing, and tees in the circuits actually in a sense "trap" fluid in certain areas of the system. Keep in mind that in braking systems, only a very small quantity of brake fluid actually moves. In fact, if you think of the brake fluid in terms of many small segments of pencil erasers, shuttleing back and forth in a row, it is closer to the actual fluid motion of a brake system. Brake system fluid dynamics are not like a pump system moving fluid around a hydraulic system.

There is no reason or gain to change to DOT 5 fluid. In fact to do so requires changing out ALL the system seals (including any internal seals in the system) and brake hoses. For street and highway riding, using DOT 5 fluid gives no performance gains over DOT 3 or DOT 4. Never install DOT 5 in a system designed for DOT 3 or 4. Believe me (experience speaking here) you do NOT want to deal with the mess that does create.

There are racing grade DOT 3 and DOT 4 brake fluids that have higher boiling temperatures closer to that of DOT 5. If you do a lot of mountain riding where the brakes are getting regularly hot enough to raise the brake fluid temperature then the racing DOT 3 or 4 may make sense. Otherwise, stick to a standard DOT 3 or 4 rating. Personally, I have gotten great brake system performance by using standard, brand name DOT 3 or DOT 4 fluid, that I change out every other year. SpeedBleeders are a great addition to your calipers to make flushing the brake easy! I do my entire 94 R1100RS (w/ABS) in about an hour.

[jsalmon]

Thanks for the response. The synthetic is DOT 3/4.

I understand that the fluid doesn't move much, but it seems that flushing out the old fluid from the reservoir would remove all of the fluid. Are you saying that there will be old fluid trapped in the servos unless I bleed them? They are difficult to get to as I will have to remove all the plastic and the tank. Even with speed bleeders this will take several hours.

[bikerfish1100]

Thanks for the response. The synthetic is DOT 3/4.

Are you saying that there will be old fluid trapped in the servos unless I bleed them?

yes, that's the reality.

[JimVonBaden1]

Thanks for the response. The synthetic is DOT 3/4.

I understand that the fluid doesn't move much, but it seems that flushing out the old fluid from the reservoir would remove all of the fluid. Are you saying that there will be old fluid trapped in the servos unless I bleed them? They are difficult to get to as I will have to remove all the plastic and the tank. Even with speed bleeders this will take several hours.

Not at all. The process is pretty simple, and with practice should not take more than 1.5 hours, with removing all your plastics taking longer than the actual bleed. Look on the Wisdom section of www.advrider.com, or the FAQ of www.bmwst.com for complete instructions.

Jim :brow

PS You can also get a video of how to do it.:whistle