Topic: Dang brakes

[eigenvector]

A while back I was asking for advice for a front brake that wasn't returning after contacting the rotor.

Everyone said "rebuild the master cylinder"
Which I finally did this morning.

Put it all back together, absolutely certified that the return hole was free, primed the system and bled the brakes.

It STILL won't retract back into the caliper after contacting the rotor.  Opening the bleed screw does nothing.

If I loosen the caliper bolts it helps, until I hit the brakes again that is.

I don't see what the problem is.  Next step would be to totally rebuild the caliper, but I don't see why that should be necessary - the caliper seal was replaced 2 years ago and the bore was clean without rusting or scratches.

To guide this in a constructive direction - pad clearance.  How tight/loose should the pad be in the bore.  Floppy, barely touching, somewhere in between?

[jonda500]

Pad should slip out easily - 'Floppy, barely touching, somewhere in between?' all of the above would work. Pump the piston out and make sure it's not gummed up in there, make sure there's no crud under the seal - the seal slot must be clean.
John 

[eigenvector]

I'll take a gander - looks like the rain has abated for now.

[ekpent]

 If the pads are new and they have paint still around the edges on the metal that can be a problem and sometimes the actual sides of the pads have to be sanded and groomed to fit nicely also.

[pjlogue]

If you rebuilt the MC and assembled everything properly and the caliper is clean and things fit properly then check your break lines.  If they are old the lining can break down and cause a flap that acts like a one way valve.  I had this happen in an old car I had.  Drove me insane for months before I replaced the hoses. 

-P.

[eigenvector]

Had to have been the piston/bore.

I stripped it smoothed everything down, wiped off some built-up grunge from around the pad edge.

Not quite as smooth as my 2015 BMW, but did notice that the piston needs to be resurfaced - has some imperfections which I knocked down with some fine grit paper, but I think later this year I'll do a more thorough job.  The bore needs to be honed, nothing earth shaking but I suspect this is why the brake is dragging.  I don't have the hone to do this now.

[kerryb]

It has been said here that the square-edged shape of the caliper bore seal is what pulls the brake piston back into the caliper, allowing the pad to move away from the disc.  So while you're in there check the condition of the seal.

[disco]

I've had really good success using this Bendix product. Brake feels great. Retracts good.

By comparison, one of my other CB750's had caliper rebuilt about two years ago. Caliper was absolutely spotlessly clean. New seal went in. I dressed/filed the pads etc. Reassembled using just brake fluid. Now leaking.

[Spanner 1]

kerryb +1.... the only thing that moves the pad away from the disc when you release the brake is the 'square' piston seal which gets contorted, or slanted off square when you pull the brake lever. Without the brake pressure the seal tries to return to square pulling the pad a fraction off the rotor. Did you remove the piston seal and clean out the corrosion under it ? If not, that's probably the cause.

[Terry in Australia]

All of the above advice is valid, and just for an "outside the box" possibility, I had the same problem with one caliper on my twin disc setup, so I did everything suggested above, to no avail. I had the pads out for the umpteenth time when I realised that there was too much "pad" material, even with the piston fully retracted, so I stuck them in my lathe and trimmed a few mm off the pads, put them back in, and have had no issues since.

At least one other member has told me that he's had the same issue, we both bought our pads on ebay, so there must have been a bad batch at one time or another. The easiest way to check would be to re-install an old set of pads, and if the problem goes away, well, there's your problem...........

[eigenvector]

Sometimes it really is that simple Terry