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, so DOT4 is used on all new 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, bleeder 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 brake failure.
• 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 a DOT5 system, the boiling point drops sharply to 100 °C. 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 bleeding, so the caliper must be removed and purged.

You mustn't mix silicone and glycol-based fluids, because they affect seals, rubber parts, 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 about whether DOT4 in a DOT5 system can cause problems, but we faced it ourselves. Alexander Andreev topped up his FXR rear brake system with glycol-based DOT5.1. As a result, the caliper piston stuck and didn't return, the pads were worn down to the backing plate 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 stuck.

In the video, we showed the caliper repair process and replacement of the brake hose and brake disc. The system was bled with a vacuum pump — the fastest and most efficient way. We didn't disassemble the master cylinder. Instead, we just flushed it with fresh fluid. But we know that if DOT5 is poured into the DOT4 system, all the cylinders must be disassembled and washed, as silicone DOT5 is very hard to flush out and can remain in the cylinders.

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