Linked brake issue

[Gearhead]

Hi. My linked brakes have been weird for a while. I posted how I found the rear caliper worn and grooved which hung up one end of the pads, and I figured I had found it. I fixed that (write-up to come) and the linked brakes are better, but...

 

When I first push the pedal, the brakes work pretty well and the pedal is pretty firm. But if I let up a little (maybe 1/8"?) and push again, as sometimes happens when you stop gradually, the pedal drops considerably, sometimes all the way down and I get very little braking until I let the pedal all the way up and push it again. This "resets" them and they work fine again.

 

I don't think this sounds like air in the system as that should always cause a squishy pedal that gets a little firmer with pumping. That's not what's happening. I'm thinking a bad master cylinder - do you agree? I don't understand the metering and proportioning valves very well, so I dunno if one of those could be the culprit.

 

FYI, I thoroughly bled the system with a Mity Vac at the rear, front and metering valve locations.

 

Help!

 

Thanks, Jeremy

Edited September 28, 2011 by Gearhead

[Gearhead]

Anybody?

[CrazyHorse]

Have you ever rebuilt the rear master?

[Gearhead]

No, only flushed and bled. No external leakage.

[skydoc_17]

Hey Jeremy,

When the MKII Rear Master Cylinder fails, it doesn't leak. Fluid that should push thru the brake lines to the rear caliper, and the left front caliper passes by the Master Cylinder piston, (worn Seal) spirals down the plunger stem, and returns to the fill line to the reservoir. (designed that way) I have the Genuine Yamaha rebuild kit for the Master. A few of the items that cause the Master Cylinder to fail are: AGE, HEAT from the engine, brake fluid that has not been changed, and one or more calipers that are dragging, putting excessive pressure on the M/C plunger seal.

A rebuild of the M/C is where I would start, but be prepared to address your 24 year old calipers as well. This could be one of those "Classic Snow Ball effect" type of repairs, where you will address one issue, and the next link in the chain will cause a problem next.

If I can help, please feel free to PM me.

Earl

[Yammer Dan]

What Earl said.