Harley-Davidson FXR Rear Brake Repair

Before the introduction of ABS, Harley-Davidson motorcycles used DOT5 silicone brake fluid. This liquid is better than glycol-based DOT 3/4/5.1 in every way, but it is aerated (air bubbles appear) when the ABS unit is operating, therefore DOT4 is used on all fresh Harley-Davidson motorcycles.

• Silicone fluid has the highest boiling point both in a clean state and with water. This parameter is comparable only to DOT5.1.
• DOT5 doesn't absorb moisture, so it doesn't require replacement, unlike the glycol-based ones, which need to be changed once a season. Glycol-based fluids absorb moisture from the air. As a result, all steel elements inside the system, such as the master cylinder piston, bleader bolts, and fittings rust.
• DOT5 is non-toxic and doesn't eat the motorcycle paint, which makes the brake system maintenance much easier.
• DOT5 is not used in racing because during hard braking it foams and the brake lever becomes soft. Together with overheating, this can lead to loss of brakes. Also, if water gets in, it will settle at the bottom of the caliper (water does not mix with DOT5 and it is a heavier fluid). When the water boils, there are air bubbles in the caliper and the brakes stop working. Thus, when water enters the DOT5 system, the temperature drops to 100 degrees. And in the DOT4 system, the boiling point drops gradually as the amount of water in the system increases.
• As water settles at the bottom of the calipers in DOT5, it cannot be removed by pumping, so the caliper must be removed and purged.

You mustn't mix silicone and glycol-based fluids, because they affect seals, rubbers, and metal coatings differently. For example, Brembo clearly instructs not to use DOT5 in their systems, because it destroys rubber seals. They also produce special repair kits to switch to DOT5.

We didn't find any information online if DOT4 in DOT5 system can create any problems, but we faced it ourselves. Alexander Andreev filled up his FXR rear brake system with glycol-based DOT5.1. As a result, the caliper piston wedged up and didn't return, the pads were worn down to the base and ruined the brake disc.

We haven't found the reason why it happened. There was something black on the cylinder walls, but the rubber seal wasn't damaged. The paint on the fittings flaked off and got into the liquid, so the liquid had clots in it. All this slag didn't allow the piston to move freely, which is why it wedged.

In the video, we showed the caliper repairing process, how to replace the brake hose and brake disc. The system was pumped with a vacuum pump - the fastest and most efficient way. We didn't disassemble the main cylinder. Instead, we just washed it with fresh liquid. But we know that if DOT5 is poured into the DOT4 system, all the cylinders must be disassembled and washed, as the silicone DOT5 is washed out very badly and can remain in the cylinders.

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