Yamaha ULTRAMATIC GRIZZLY 660 2001 Manuel Du Propriétaire page 364

Brake fluid replacement
Complete fluid replacement should be done only
by trained Yamaha service personnel. Have a
Yamaha dealer replace the following components
during periodic maintenance or when they are
damaged or leaking.
G
Replace the oil seals every two years.
G
Replace the brake hoses every four years.
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EBU01186