Topic: Bottoming out front brake lever CB750K5

[PxTx]

Helping my sister get here CB750K5 on the road.  It has medium cruiser bars, so the MC angle is not ideal.  The bike is new to us, so I don't know if there is a bad component in the system, but the brake lever can be bottomed out.

I have bled the entire system, opened every banjo, bleeder, brake line and removed tho MC from the handle bar to get the ideal brake line routing for bleeding.  I got just a few bubbles out of the MC, but since then, nothing but fluid.  Brake is still squishy.  I even took the caliper off and pushed the piston all of the way back.  It took 5-6 pump to get the piston back in place.  Pad looks to be a little thin (maybe 30-40% left) but it has been a long time for me to reference what it should be.

Does anyone have any thoughts?  I don't think there is any air in the system.  I did search the forum and saw a post about a pad being too thin.  I don't think this is THAT thin where the lever can be bottomed out.  Could there be a MC problem?  I have another MC to swap on, but I wanted to hear other ideas/theories on what might be going on here.

[manjisann]

Check and see that all the holes in the MC are clear, there is an itty bitty hole that allows the fluid to return into the MC. I beleive if this is blocked it usually causes the caliper to stick and locks the brake, but doesn't hurt to check. If you are 100% possitive there is no air in the system, then a MC rebuild might be in order. Something you could try if you haven't already, get a clear peice of tubing (the kind they use in fish tanks for air hose) and stick that over the bleeder, run it up and over your handle bars so it is at least as high as the MC. Open the bleeder and allow the fluid to just flow up it. You will need to top off the MC periodically as any air getting in would just set you back to square one. Since you have to buy more brake fluid that our bikes hold, pump a bunch through, it doesn't take many bubbles to cause what you are describing.

HTH.

Brandon

[Spanner 1]

If your brake lines are old they will expand when the pads have contacted the rotor as you pull the brake lever.....hold the bottom line half-way and pull the lever hard....does the hose expand ?...my 2c...

[BeSeeingYou]

The bike is new to you so you don't know how old the seals are and the brakes did not work when you got the bike.  It is 33 years old so don't scrimp on safety. At a minimum rebuild the MC and put a new seal in the caliper.  About $35 in parts.  New brake lines would be nice if you want to spend another $50 or so.

[PxTx]

Thanks for the input.  I hear what you are saying abut replacing any unknown saftey related parts.  I would just like to identify the faulty part of the system before i go redoing everything.  It would be my luck to replace everything, then induce more problems. 

So it is not uncommon to have a MC generate some pressure ,but bottom out the lever before additional brake pressure can be generated?

It is less likely that the pads are low, right?

[Spanner 1]

Even if the pads are 'down to the metal', the piston will just move out to compensate for the ware AND keep the pads at the same distance from the rotor as if the pads were new....
Also a worn cup seal in the m/c will allow fluid past it under pressure, i.e. allow brake lever to pull to grip....not normal for any brake, ever !

[BeSeeingYou]

Did you get any air bubbles out of the caliper?  You may still have air in there somewhere.  It seems you are developing some pressure with the MC as it did push your piston back out but with the caliper loose this does not take all that much.  Even a leaky MC would be able to do it.  Once mounted on the bike it's a different story as it needs to develop much more pressure.  The parts that wear out and cause problems are the caliper seal and the rubber MC cups so there is nothing to lose in replacing them.  Sometimes you can have a badly pitted caliper piston too but the caliper and MC castings are usually not the source of these problems.

[PxTx]

I thank everyone for the input.  I suspect the MC cup seal is leaking pressure.  There was a little bit of fluid around the pivit point of the lever and I'm pretty sure I did not spill any. 

If I clamp and hold the lever, it seems to keep the same firm pressure for as long as my hand will hold (about 1 min).  So the leak if very minor.  It seems like it is capable of generating some pressure, but not 100%.  I would say maybe 35-40 percent pressure.

Tomorrow I am going to swap on another unknown MC.  If it works great.  If not i will then rebuild the original one on the bike (cosmetically it is nicer).  Thanks!