Break drag on an FJ

I took the front end of a 2002 R1 (wheels and forks) and put it on an '86 FJ 1200. For whatever reason I can't seem to figure out the brake drag that's happening. I completely took apart the master cylinder and thoroughly cleaned it. I went through the brake lines with carb cleaner and guitar strings. I popped out each piston in the caliper to make sure they were running smooth. Everything looks good. The only difference in the set up is that the master cylinder is different as it's the original off the FJ. Also, the brake lines go directly from the MC to the calipers rather then one brake line extending from the MC into a joint that divides into two brake lines going into the calipers. Any suggestions?

 

did you replace the seals on the calipers?

both bikes are supposed to have single line going down and splitting off to 2 lines

 

I’m not familiar with that front end but I’ll assume there are slides on the calipers. Are they clean and greased?

No problem with running two lines down from the MC as long as you have the correct banjo bolt and washer between then lines.

Are the master cylinders the same bore? I doubt that would cause it to stick but from experience I know a subtle change in size can make for a huge difference in braking.

Did you make sure the bleed hole in the master is clear? They can be very very small.

 

I completely tore down the master cylinder and cleaned it. Ran a piece of guitar string through the bleed hole. The calipers have 4 pistons each. I popped out and cleaned each one and only one was a little bit stuck. I did not replace the seals but the pistons moved so freely I almost thought they were too loose!

 

I'm not sure about bore sizes but I think the brake line set up my be the culprit. With two long brake lines coming directly from the MC maybe it's not creating enough back pressure to pull the pistons back. Every bike I've seen that has dual calipers has a single brake line coming off the MC and then splits into two. Maybe that subtle change is screwing everything up.

 

I am no hydro engineer but I don't think there is any pressure pulling the pistons back. Once the pressure from the MC is removed by letting go of the lever, the fluid returns to the MC and the pistons should return to a "off brake" state.

I would guess that something is hanging up on the caliber setup. Can you grab the calibers by hand and move them side to side to free up the brakes? If not then you need to polish the pins in the calibers and or grease them better.

 

There is a little "pullback" of the fluid from the MC piston until the relief hole is exposed allowing any remain travel to be made up with fluid from the reservoir. It's not much and barely noticeable but needed. I'm wondering if the MC is not correct since there are 8 pistons total, there may not be enough to pull all back therefore causing sticking.

 

the piston seal deforms a little as the piston gets pushed out, when the pressure is relieved the seals try to return to original shape and they pull the piston back with them.
yours, being fresh and all lubed up might let the piston slide out without deforming the seal, so nothing pulls it back.
how tight is the wheel?

 

Sounds right, just still wonder if the relief hole is open, then any pressure in the system wold be passed back to the MC reservoir. Even the movement of the brake disks moving around and pushing on the pads should move the pucks back if there is no pressure on the brake lines.

I wonder if you can take a flat screw driver and push the pucks back by hand? Don't hurt the brake pads doing this but see if that loosens up the pads.