Why's my FZ1 rear brake useless?
Q. The rear brake on my Yamaha FZ1 is useless. I told my local Yamaha dealer when they had it for its 12,000-mile service and afterwards they said it was OK. They said the pads looked almost new (they would do as they don't do anything).
On anything other than level ground I have to hold the bike and pull away off the front brake.
Alteda, MCN forums
A. Try taking the pads out and giving them a rub with some medium grade glasspaper to deglaze them (take the shine off). Also your rear caliper is a floating type, if it isn't sliding freely on the bolt sliders (no grease or starting to seize) on the carrier, then this could also stop the brake from working properly.
You should check the caliper piston is clean and moving freely. Look at the linkage too, that the pedal is pivoting freely and the rear master cylinder push rod is moving freely with no corrosion or rust present. Also bleed the system right through withfresh brake fluid.