Topic: 78F 750 rear brake loss of power (fixed for now)

[cheftuskey121]

wanted to come here first before I do anything to see if anyone has insight. new bike, new model. I am not too familiar with the rear disc brake and its parts/operation. it was working just fine yesterday. it worked fine this morning on the way to work. left work tonight and noticed the rear brake was essentially useless. no braking power whatsoever. nothing out of the ordinary changed, I did not hit it or anything like that.

when I got home I could see there was fluid in the reservoir, no leaks at any of the junctions, or punctures in the hose. when actuating the brake it will stop a hand-spun tire at maximum pressure. so it is doing something. but its useless for the road and I won't ride it until I get it sorted. could this be some kind of adjustment issue? maybe even a super worn brake pad not having enough material to make contact?

if it is a sealed (theoretically) system how can it just fail from the inside out. looking for insight and things to check before I jump in TOO deep. rather take my time and be methodical then try and rip everything apart. I KNOW new brake pads all around wouldn't be the worst thing in the world. hope you guys read my whole post, I know I type a lot

thanks!
-Justin

[scottly]

Does the pedal go all the way down? If so, the brake system may need bleeding. If the pedal is firm, check the rotor and pads for oil or other contaminants. You should be able to tell if the pads are worn by looking at the thickness of the friction material. The F rear brake is basically the same type of hydraulic system as the front brake, and the same rules apply.
My guess is that since it worked in the morning, but not later, is oil from the tank is leaking onto the brake. Just a WAG....

[cheftuskey121]

scottly, the brake pedal goes ALL the way down yes with barely any power. it feels soft, like there is air in the line or something....but HOW can that happen when there was no physical damage or change. no leaking of brake fluid. I don't think there is oil on there but I can get some brake cleaner and at least rule that out.

[andrewk]

Could be a leaking sealing washer sucking a bit of air.  Could also be master cylinder internals sucking air too...  What's the history on the bike?  These are simple brakes, but often suffer from neglect. Might be time for a rebuild.

[Thousandson]

tear into it. Mine was a problem with the return hole on the cylinder. Since that was plugged I couldn't bleed my brakes properly. And yes, get all new copper washers. I had one leak on my front end, and just replaced them all. Cleaned, bleed and running great.

[seanbarney41]

sounds like a bad internal seal in the master cylinder....if it will pump up pressure(pump the pedal rapidly), you are getting air in the system somewhere, and the air leak needs to be fixed and the system bled...

[cheftuskey121]

sounds like a bad internal seal in the master cylinder....if it will pump up pressure(pump the pedal rapidly), you are getting air in the system somewhere, and the air leak needs to be fixed and the system bled...

thanks guys. when it is pumped up it functions properly so yes there is air somewhere. how can air get in, old rubber hoses? or sneaking past caliper?

[andrewk]

Air could be seeping in through the hose, or through a bad/dirty master seal, or a sealing washer.  Air doesn't usually leak past the caliper without fluid (and a hanging brake) too.   You're going to need to disassemble both the master and the caliper to inspect, clean, and replace any worn rubber parts.  Everything you need (except the caliper piston and brake hose) is available from Honda, the piston and hose you can get from forum members here if you want something better than OEM. (recommended) 

[Thousandson]

If you want to spend the money, I did for my piece of mind. Sling shot motorcycles, makes a killer set of stainless steel hoses. I got a set of hoses from him, great quality of work, snapped into place, different lengths for different handle bars, he labels all of the hoses (no need to think twice about where hoses go). The only thing that I didn't like, is that it might take him a two weeks to make the hoses. The wait and the money was worth it.

I just looked into his brake line collection, and its gaining momentum, and variety. I can find a new stainless steel hose for my cb450 project, I can't wait.


http://slingshot-cycles.com/

[cheftuskey121]

Yeah I have slingshot stainless hoses on my k1. That was the plan for this front an rear as well anyway. Looks like it's time for some work. Thanks.

[andrewk]

If you find your piston is pitted, I am a big fan of Godffery's phenolic replacements.  A bit more money, but you'll never buy another.  http://www.vinmoto.org/gallery/v/VinMoto-Garage/album03/Godfferys-Garage-123/95/PHENOLIC+Replacement+brake+Caliper+piston/PHENOLIC+Replacement+brake+piston+for+Honda+CB+650+-+750+DOHC/

He calls those brakes "79-81" but they came out in 77 on the F2, should be "77-81," fwiw.

[cheftuskey121]

If you find your piston is pitted, I am a big fan of Godffery's phenolic replacements.  A bit more money, but you'll never buy another.  http://www.vinmoto.org/gallery/v/VinMoto-Garage/album03/Godfferys-Garage-123/95/PHENOLIC+Replacement+brake+Caliper+piston/PHENOLIC+Replacement+brake+piston+for+Honda+CB+650+-+750+DOHC/

He calls those brakes "79-81" but they came out in 77 on the F2, should be "77-81," fwiw.

thanks andrewk. I looked at those before but never pulled the trigger. probably will on this build. I bled my rear brake and its functioning again. I know its not the proper fix, just a bandaid, but I dont really want to take the bike off the road just yet until my K1 is rebuilt. I will keep bleeding it until it won't bleed anymore.

[Stev-o]

Hey Chef - you obviously have an issue, air is getting into your system somehow.  Odd that it was good one day and not working the next.  You sure you don't see a fluid leak?