Rear Brake Failure.

This morning on my commute, while coming to a nice controlled stop at a light, my rear brake pedal slowly traveled to the end of its stroke. The rear braking simultaneously faded to nothing in the process.

Assuming a leak in a brake line or something I discontinued use of the rear brake to avoid squirting fluid on my rear tire or anything else.

On arrival at work I cycled the brake lever several times expecting to see fluid weeping out somewhere. None to be found. I did notice a slight squirting sound coming from the master cylinder/reservior area when the pedal was cycled which turned out to be fluid squirting back into the reservior.

I think this must be a failed master cylinder piston seal but i cannot find any threads to confirm. Has anyone dealt with this problem on an oil head? For reference, mine is a '96 R1100RT.

Gotta get this fixed before the National!

Thanks in advance.

I haven't dealt with this, but I think your diagnosis (failed rear master cylinder seal) is correct.

When was the last time the rear brake fluid was replaced? Or for that matter, has the rear brake system ever been flushed with clean brake fluid?

Could be a combination of water ingression, old brake fluid and a failed seal at the caliper or foot pedal master cylinder.

If the caliper has anything more than the normal brake dust buildup in it, it could be a very slow leaking caliper piston seal. The fluid would mix with the brake dust to form a pastey kind of buildup.

In some cases it could be the brake hose itself is so aged that the hose casing reinforcement (polyester yarn in most OEM brake hoses) has broken down and failed internally. When pressure is applied the hose itself is expanding so much the brake can feel very spongy.

+1 on the Brake fluid issue. This happened to me on my 1100, when I first had it I was testing - learning about braking capability etc. and I was riding into town probably never exceeding 25 MPH and the brake pedal seemed to go down further with each stop finally no brake at all. Could see no leakage of find anything obviously amiss. Went into a store did my errand no more than 3 minutes and fired up the bike to ride home - the rear brake functioned fine. Did your brake come back after sitiing? Loaded the bike on the truck - truck on the boat 2.5 hour boat ride - 90 min drive to the dealer (used bike warranty). Had a couple of other things fixed that I had found and told them about the brakes. When I came back they had fixed the other items but said they could not make the brake fail ie "nothings wrong". Brought the bike home duplicated my ride and no problem then or ever again. I had read somewhere that the fluid if left unchanged attracts water and under hard use the water boils and the brakes fail so I suspect they changed the fluid ( why can't they just tell you that?). anyway good luck hope you solve your problem.

Found The Problem!!

The cup seal within the master cylinder had a large crack in the bottom. The crack allowed fluid to vent from the compression area in the cylinder to the area which supplies new fluid from the reservior. That created the pressurized return of fluid to the reservior (squirting I observed).

Looks like the part would cost pennies to replace. I will be rebuilding the cylinder ASAP.

BTW the brake fluid is meticulously maintained. Flushed annually.

Thanks for the detail on your reply. Now I learned something too. If that ever happens to me I'll know what to look for and expect.
Andy