Question about the rubber diaphragm in the MC

[Guest]

I have been chasing a soft brake lever issue ever since I rebuilt the MC and installed stainless steel lines.

I have thought that I got it fixed about a dozen times, but the brake lever travel returns to a long pull each time.

I rode for about an hour last night, pulled into the garage and attached my vacuum bleeder immediately. I am NOT pulling any air bubbles in the fluid...but I still have long lever pull.

The only part I haven't replaced is the rubber diaphragm in the MC. There is no obvious damage to the rubber and it appears flexible.

What would the symptoms be if the seal is not holding at the cap?

[bwringer]

If this rubber piece is bad, the symptoms would be that brake fluid leaks out.

In other words, it's very likely not related to your problem -- the reservoir is just a reservoir -- it's not pressurized. The diaphragm is just there to keep the fluid in and outside air out, and it expands in order to let air in as the pads wear and more brake fluid travels to the caliper. You'll notice there are tiny air channels in the cap to accomodate this.

When you spin the front wheel with the wheel off the ground, does the brake disk look like it's wobbling side to side (runout)? If it is bent or mounted a little off, it can push the pads back into the caliper as the disk rotates.

[Nessism]

When bleeding it sometimes helps to tap on the lines, including the junction block if you still have it attached, so any clinging bubbles release. Also, you might want to try tying back the hand lever as tight as you can and leave it over night. Sometimes that helps pull bubbles out and force the caliper pistons closer to the discs, thus firming up the lever.

BTW, if your master cylinder is small diameter that will create a spongy lever feel. Dual disc masters should be 5/8" and singles are 14mm.

[Steve]

Which diaphragm are you talking about?

The one that bwringer mentioned, that is just under the cap, or the one on the piston, that actually pressurizes the fluid?

If you are talking about the latter one, if it's weak, you would experience the symptoms that you are having.

.

[oldrookie]

Thanks for the help, gents.

My question was concerning the under-the-cap diaphragm.

I tied the lever back before work this a.m. after I bled it again. It firmed up nicely, but it has done it before. I worked to tightened the banjo bolt at the MC as well.

Found some fluid on the underside of the bolt when I got home a few minutes ago. Some evidence of fluid coming from under the cap, but I'm about ready to bite the bullet and get a used MC off a GS500. A guy on advrider has swapped his out and it worked.

If it is wet again in the a.m. without tying the lever back, I'll replace the whole deal.

Again, thank you for the advice and help