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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

DOT 5 Brake Fluid

Motorcycle Classics tech expert Keith Fellenstein

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DOT 5 brake fluid

Q: What do you think of using DOT 5 brake fluid in a completely cleaned and rebuilt brake system on a 1979 Yamaha? The caliper kits were recently purchased. The master cylinder kits were NOS Yamaha. The lines were new Goodridge. No DOT 3/4 (or 5) has been in the rebuilt system as yet. I rarely ride this bike. Any thoughts? — Steve Haigh/via email

A: Of all the various brake fluids, from DOT 3 to 5.1, DOT 5 is the only one incompatible with the rest. It is silicone-based where the others are glycol-based. That’s good news for water absorption and paint protection. I haven’t changed any of my old systems from DOT 3, but they get ridden and checked pretty often. DOT 5 is supposed to be more stable and less interested in pulling moisture from the air into your hydraulic systems, so it is probably better for long-term sitting than the old hydrophilic fluids. However, since it won’t absorb water, and is lighter than water, any water that finds its way into your brake system will pool at the lowest point, the caliper. This isn’t good for corrosion or for brake operation, since the caliper will get hot, and the water will boil long before the brake fluid. Also, everything I read gives dire warnings of seal damage and brake failure if there is any cross contamination with old glycol-based fluid. If you’re 100 percent sure your entire system is clean (including any pressure operated brake switches), I suppose you’d be OK to make the change, but I can’t see any real advantage. MC