Rear brake delamination and it's front brake counterpart!

I've noticed the rear brake shoe warning several times. I'm glad people are being warned and paying attention to this. My own were completely separated and fell right out when I removed the wheel. I haven't ridden the bike yet but this is something I always check. I work on vintage bikes of all type and size daily, and this is a VERY common problem on vintage bikes. If the brakes are old, or if you're not sure how old they are, just replace them. They may look fine when you check them, but that doesn't mean they won't suddenly give up the ghost somewhere down the road.
(I couldn't post this on the original thread, though I'm aware that the point has been run into the ground. Couldn't help myself.)
Now, onto the front. I'm not sure if anybody has mentioned this yet, but there is an equally, if not more, dangerous demon that lives in front hydraulic systems. Think about this: When you apply pressure to the lever of your front brake, that pressure travels down the line to the caliper and moves a piston, which forces the pad into the rotor, slowing you down. When you release the brake, that pressure then has to return to the master cylinder and back into the reservoir through a TINY pinhole going into the reservoir. Imagine a carburetor jet. The pressure relief hole is about that size. And, just like an old carburetor jet, the pressure relief hole can get clogged up. Brake fluid breaks down and gets gummy over time, clogging the hole. What this means is that your brakes may work just fine for a little while, but suddenly they decide to stay on after you left off the lever. It may be no big deal. At higher speeds, or if you don't notice, it can drag enough to quickly cause enough heat to lock up completely. I've heard stories and talked to lots of people who have had this happen to them on vintage bikes. It happened to me once on a 2000 Aprilia RSV that had been poorly maintained and spent it's life in New Orleans. Bleeding vintage brakes is not enough. Rebuild them COMPLETELY, paying special attention to this detail. Seriously, this happens all the time. I'm sure many people will back me up on this. Neither of these problems are model specific.

 

The Fluid is not supposed to return to the Master Cylinder Reservoir.

When PRESSURE is applied the MC's MAIN Seal forces the Fluid in front of it, ... down into the lines and caliper causing the Caliper Piston to compress the Pads against the Rotor.

When the Lever is Released, ... the PRESSURE is relaxed and the MAIN Seal return to its at-rest location.
A Minimal amount of Fluid returns to the Reservoir because the Main Seal is a ONE WAY Seal.

The Replenishment Orifice allows Fluid from the Reservoir to REPLACE the Fluid in the System as the Brake Pads wear-down of a Caliper Seal leaks.

There are Metering Orifices drilled in the Activating Plunger to allow the MC's Secondary Seal to force Brake Fluid around the MAIN Seal as needed.

Any large volume of Brake Fluid returning to the Reservoir indicated that the Main Seal is defective or the Bore is pitted allowing Fluid to retreat beyond the Main Seal.

 

Jimbo is absolutely correct. There is a return orifice, that if restricted will not allow pressure to be relieved. Have multiple service manuals that warn of this issue when servicing the master cylinder...Honda and Kawasaki. I have also personally experienced this on an 83 Honda VF750F. The front brakes had locked up and would not release. Rebuilt the calipers, but the problem persisted. Rebuilt the master cylinder, everything looked clean so I reassembled with the new parts, but the problem was still present. After disassembling the master cylinder a second time, found a small piece of rubber (apparently from the old master seal) logged in the return orifice. After a thorough cleaning and re-assembly, the brakes worked perfect.

Ironically, Motorcyclist (January 2013) MC Garage on page 88 has a small question and answer article detailing this exact issue.

 

http://xjbikes.com/Forums/viewtopic/t=18377.html

 

The port in question is called the compensating port, named due to the fact that brake parts get hot and expand and the fluid then needs to return back to the reservoir via the compensating port.

I teach this to apprentice mechanics, but mainly due to the fact on cars, if the pedal is adjusted to a point that there is no freeplay, the port may be covered causing the brakes to lock when they heat up.

 

Well said. Thirty year old lines need to go too.

The biggest issue is that quite often, they LOOK OK, and seemingly work OK. It's not until you start trying to ride the bike that the problems will surface, and it can be with disastrous results.

A lot of folks that get ahold of an old bike are trying to get away with spending as little as possible to get it on the road. Unfortunately, when it comes to old brakes, that's simply dangerous. Thanks for posting, the original thread got locked when it got moved.

 

The Master Cylinder's MAIN Hydraulic Seal is a "One-way" Seal.
Once Brake Fluid gets on the Pressure Side of that Seal, ... it does not return to the Reservoir.

The Replenishment Port keeps the Volume of Brake Fluid between the two seals at a constant level.
As Pads wear-down the Fluid between Seals gets Pumped through orifices drilled in the Piston and seeps past the MAIN Seal replenishing the Volume decreased by Pad Wear.

The Hydraulic Pressure supplied by the Secondary Seal causes the Brake Fluid to seek the "Path of Least Resistance"
First, forcing Fluid through the Orifices in the Piston to balance the volume in the System compensating for Pad Wear (or leaks).

Then, through the Replenishment Orifice to allow a slight amount of Fluid to escape back to the Reservoir to prevent blowing the Secondary Seal.

It is when the Main Seal loses it One-way Pumping ability that a large volume of Fluid is forced back into the Reservoir causing a fountain-ing of Fluid from the tiny orifice, ... indicating that the MAIN Hydraulic Seal has failed does a large volume of fluid re-enter the reservoir.