Rear brake problem ??

[maury]

'75
I totally rebuilt bike less than 500 miles ago. All new brake components including braided steel lines.
Out riding today. Gassed up and when leaving the gas station, rear brake pedal went all the way down with no resistance.
Rode about a mile and kept pushing on rear brake pedal with no braking. Then suddenly, the rear brake was working normally with no problems. Rode and tested numerous time.
Back in the shop, no leaking fluid, removed reservoir cap and fluid was completely full.
What the heck?? Makes me kinda nervous if you know what I mean.

[redglbx]

Probably air in the system, I would bleed it, there are numerous places that trap air bubbles, I would rebleed the fronts as well.

[cfairweather]

If bleeding them doesn't work, remove and disassemble the master cylinder. Make sure the cylinder is smooth and clean. Make sure the small holes from the reservoir to the cylinder are not blocked. I would install a new kit with fresh rubber unless you already installed a new kit 500 miles ago. Make sure you assembled it correctly by looking at pictures in the manual or parts diagram.

[Sidecar Bob]

Did you check that the rod that pushes the master cylinder when you press the pedal is properly attached to the master cylinder? The original one on my '77 had some corrosion and the circlip that holds the washer in used to pop out. The solution was to replace the whole master cylinder.

[maury]

Thanks for as always your excellent suggestions.
When the episode happened it was really weird. The petal bottomed out with absolutely no resistance (so much so that the spring was pulled out of the switch). Then about 8+ tries with nothing..........suddenly complete and perfect rear brake.
I did check the attachment points and all seems good.
Tonight when looking it over I discovered the rear brake is dragging slightly. I am cheating to the safe side and rebuilding the master cylinder and as well checking out the pistons.

[cfairweather]

Glad you are going to rebuild the master cylinder. Remove the plastic reservoir so you can properly clean the crap out from underneath the bowl. It makes it easier to clean out the small holes too. I use a rotary buffer tool and Mother's Metal Polish on the cylinder. I also sometimes put the parts in my ultrasonic cleaner.

[redglbx]

Mothers is a mineral oil based product which is really bad for brake system seals, any residue will kill the brake seals.

The clue here is the dragging rear brake so either you have a piston sticking in the caliper or probably the spooge hole in the m/c is plugged. The spooge hole is a very tiny bleed hole in the m/c that is easily plugged, it is located under the plug/nut on the side of the m/c if memory is correct. Use a small wire strand to clean it out. Do not drill it out, although you can use an appropriately sized drill bit to push the hole open.

And finally, only use clean brake fluid to clean or lube brake parts. Or a proper brake system cleaner.

[cfairweather]

It is only to polish the cylinders. You clean it all out before reassembly. If someone is so dumb not to get everything perfectly clean before reassembly, they shouldn't be doing this task in the first place.

[Sidecar Bob]

When you clean it out how long do you soak them and in what solvent? Polishes like that work by soaking into the surface of the metal so just wiping it won't get it all out.

[cfairweather]

I never said I soaked them in anything. I use brake cleaner or put the parts into my ultrasonic cleaner. Never had any issues.

[Sidecar Bob]

I didn't say you soaked them. I was implying that a solvent soak is the only way to reliably strip oil from aluminum.

[cfairweather]

I would disagree with the notion that soaking aluminum in a solvent is the "only" way to reliably get the oil out. I am confident that brake cleaner spray or ultrasonic cleaning does an adequate job.

[Sidecar Bob]

I wouldn't take the chance. But then again I wouldn't find it necessary to polish the bore of a master cylinder either.

[cfairweather]

Are you serious or do you just like to argue?

[redglbx]

CF, obviously you know about mineral oils ruining brake fluid seals, my intent was to make sure anyone reading this would as well. Brake Kleen, Isopropyl or an ultrasonic cleaner should all do the job, just make sure the stuff in the ultrasonic is not a mineral oil based as well.