[Kevindpeiza]

I was out riding about two days ago when I started to feel like my rear brakes was binding and the bike was holding back .I pumped on the brake peddle and after a while it stopped but then I no longer had rear brakes. Got back home safely checked the bike out. The Brake pads was not warned out the brake fluid was still at max but no brake. I took the bike to the mechanic and he bled out the old fluid while replacing it with fresh ones He even changed all the seals in the cylinder and bled it again but the brake is still spongy and not holding. any help or advice would be greatly appreciated.

 

[ctxdawg]

howdy, when you find out what it is, let us know because i think that may be fairly common for the ctx
ken

 

[mzflorida]

Tough to say remotely but I am willing to bet it is the master cylinder or its piston cups. When the cylinder cups block the ports, the brakes go hard. When they wear, creating the pressure required to stop becomes difficult to produce or can be produced but cannot be maintained.

 

[Vulcan Mediator]

Is that a normal wear and tear issue that all bikes will likely encounter or is less common?

 

[mzflorida]

It is a normal function of wear...in any pump really....and the potential is generally there but it won't necessarily happen.

 

[Kevindpeiza]

Tough to say remotely but I am willing to bet it is the master cylinder or its piston cups. When the cylinder cups block the ports, the brakes go hard. When they wear, creating the pressure required to stop becomes difficult to produce or can be produced but cannot be maintained.

Well I changed all the seals in the master cylinder but the problem remains the same.

 

[Freedom]

Well I changed all the seals in the master cylinder but the problem remains the same.

By seals, did you mean the primary and secondary piston cups of the master cylinder?

 

[mzflorida]

Found this diagram. It might help. Are the cups what you replaced? If not given it a go. Usually a pretty reasonable DIY project. But you'll have to flush and refill. Those black rings on the rod are the cups. Usually the rods, fully assembled are very inexpensive. If those are weak or worn you can build but not hold pressure. Remarkably simple system for what it does.

 

[jmc]

That's a good diagram of a master cylinder - thanks.

I've always rebuilt master and wheel cylinders without much trouble, but you must be scrupulously clean and use copious amounts of brake cleaner. But one time I just could not get the system bled properly and finally had to take it to the dealer here with a vacuum bleeder because - and this is the point - the ABS with its valves, nooks and crannys just wouldn't get completely rid of the air in the lines when I pumped the brakes.

So . . .do you have an ABS bike?

 

[mzflorida]

That's a good diagram of a master cylinder - thanks.

I've always rebuilt master and wheel cylinders without much trouble, but you must be scrupulously clean and use copious amounts of brake cleaner. But one time I just could not get the system bled properly and finally had to take it to the dealer here with a vacuum bleeder because - and this is the point - the ABS with its valves, nooks and crannys just wouldn't get completely rid of the air in the lines when I pumped the brakes.

So . . .do you have an ABS bike?

Those are all excellent points that i completely took for granted as understood. Great callout.

 

[Freedom]

Piston cup failure in the master cylinder would cause both symptoms - spongy and brakes not holding. Fluid may be leaking past the cups when the brake is applied. If the master cylinder was overhauled with new cups and it is still happening, maybe the cup lips accidentally turned inside out when installing or there is scoring in the cylinder.

 

[mzflorida]

Piston cup failure in the master cylinder would cause both symptoms - spongy and brakes not holding. Fluid may be leaking past the cups when the brake is applied. If the master cylinder was overhauled with new cups and it is still happening, maybe the cup lips accidentally turned inside out when installing or there is scoring in the cylinder.

I may be misunderstanding. Are you saying that whether one cup fails, or all cups fail simultaneously that both symptoms will present? Brakes hard then releasing? Or that either may happen? I am only familiar with one or the other happening.

 

[SteveDCT]

Ill just toss in one thought on this. I am in the process of changing a brake caliper on a van that locked up and previously did it on a pickup. They locked up because the brake fluid was replaced by mileage and these vehicles werent driven much so it got old. Moisture gets absorbed over time and causes piston corrosion which makes them lock up. Probably not good for other brake parts.

So I am now changing my brake fluid (also in the clutch) myself so i can be sure it is fully flushed in ALL of my vehicles and motorcycle and will do so from now on every 2 or 3 years regardless of mileage.

Steve

 

[Freedom]

I may be misunderstanding. Are you saying that whether one cup fails, or all cups fail simultaneously that both symptoms will present? Brakes hard then releasing? Or that either may happen? I am only familiar with one or the other happening.

Kevindpeiza last stated he had a spongy brake that wasn't holding. He was no longer experiencing a binding brake and didn't indicate that it brakes hard then releases. If fluid is leaking past the primary cup while applying the brake pedal, then pressure cannot be maintained on the brake caliper which would make the brake feel spongy. Plus, as the pedal is fully depressed and the master cylinder piston reaches the end of its travel, the brake pressure decreases as fluid continues to leak past the cup and the brake loses hold.

I agree with jmc that air may be the culprit. Before looking into the master cylinder, bleed the system again. I recently changed brake fluid and it took a while before I was able to purge air from the system and have the brakes work properly.