Spongy Front Brake

[The Mighty Gryphon]

My "new" 100RS 16v has an issue with the front brake master cylinder.

Just finished installing new pads and lines, and flushing and bleeding the system.  ABS has been removed.

Originally had spongy feel in brakes.  Did all the work and spongy feel is still there.  Grab quick and lever hits spongy resistance quickly and sort of holds.  Pull in slowly with little pressure and lever will slowly go the the grip with a slight resistance.

I suspect the problem is with the master cylinder, especially since I found a small amount of fluid in the cylinder's boot.  I have heard that I can have the cylinder sleeved with stainless steel.  OK, where can I have it done?  Any idea on what it would cost?  I like the sound of it since it is supposed to be cheaper than buying new, and is a permanent fix.  Seems to be a fairly common fix among the classic car guys.

[Elipten]

Problem is air in system.  Had same last year when rebuilding my K75.  Google it, but need to bleed then hold lever back to grip, I used a bungee cord, and let sit overnight.  You have to turn bars in one direction but I cannot remember which way.  Bleed again until hard.  Worked on my K75 when no amount of bleeding even with vacuum system worked.  Shit design issue obviously trapping some air in lever master cylinder.

[The Mighty Gryphon]

It's worth a try.  With 4 feet of snow in the front yard I have plenty of time to try it with the bars turned in both directions. 

I suppose that I will be doing this with the reservoir cover off and looking for air coming up from the cylinder, and not to be pushing the air to the caliper bleeder.

[Elipten]

As I recall I turned bars left, no bubbles seen.  All it takes is a tiny bit of air and it is mushy.  I was surprised it worked myself.  But it did.

[The Mighty Gryphon]

Went out and bled them again this afternoon.  Got a some very small bubbles both at the reservoir and the caliper bleed port.  Lever feels a bit firmer and makes me think I'm making some progress.  Only 30 degrees F, so the fluid is a bit syrupy.  Seems to help flush air from the caliper, but makes it tough at the reservoir.


Closed things back up and will make a note to bleed again per your advice when things warm up a bit more.

Thanks.

[Elipten]

Good luck

[kris]

Gryph, I think we were closer to 36 today. Get 'er out in the sunshine...that'll warm things up. Supposed to be a beautiful week. You should come up to Port Dover for the Friday the 13th bikers gathering!! I can't imagine that 10 or more degrees would affect your brake fluid that much. Boy, it seems an odd problem that the master cylinder traps air...but everything has its quirks. Let us know how you make out, would you? My braking system (non ABS) is completely apart and off the bike so I'm starting from square one. Good luck.

[The Mighty Gryphon]

Kris, Still too much salt out there to take the bikes out.  Need a couple good rains after the plow piles melt.  Do they still have the lake perch fish frys in Port Dover?  Good excuse to battle the traffic on highway 3.  Of course, with the lake iced over there isn't any fresh perch.

I'm finding out that these bikes aren't all that easy to bleed after the system has been taken apart.  Lots of places for air to hide I guess.  Just don't plan on getting it done in 10 minutes.  I spent about an hour screwing around this afternoon with a system that I thought I had bled.

[552255]

I have had to rotate the master cylinder on the handlebar, to get the last bit of air out...you can see the line of bubbles if your watching.   Loosen the lock screw just enough to rotate the m. cylinder, while the cover is off (carefully).  Sometimes just leaning/turning the handlebars will accomplish this.....

[Motorhobo]

The process as I remember it is bleed, turn bars full left, engage lever 1/4 slowly a few times till bubble appears, turn bars right, repeat, turn bars left, repeat, turn bars right, repeat until no more bubbles appear.

apparently a tiny bit of air stays in the chamber and this gets it out. All I know is what when I've done it, I've seen the offending bubble escape, and I've not had any sponginess after bleeding.

[Jumpin Jimmy B]

 Helpful hint.

 Repeat after me. Never ever pull the brakelever PAST the normal range of motion when bleeding the brakes on these bikes.

 Why? Exceeding the normal piston stroke can introduce all that cylinder bore oxidation into the system. Doing that can also ruin the piston seal or score it and give you a leak. This is only on a system where the bore was not just polished & cleaned. If you dissassembeled it all and polished the cylinder bore and replaced the piston & seal, then carry on & full stroke away.
 If I'm going to work on the front brakes, like replacing the lines or pads or the annual flushing of the system. I first find a small block of wood that limits the brake lever
movement to the bars to normal operational range of movement. I duct tape that in place before I begin to limit the travel to teh normal operation travel of the brake lever and then begin.
 I found this out by experience.

[johnny]

greetings...

jumpin jimmy b is absolutely correct... all kinds of whack happens when that piston is forced back beyond normal...

the po of the brick of the corn did just that... pulled the lever to the grip on a bleed... master started leaking... took a new piston and seal to fixt it...

piston link... note for ng... not a rebuild kit... o no... just a piston... ha ha ha...



j o

[The Mighty Gryphon]

Ouch!

[subforry]

A few syringes are best brake bleeding items ever.  You can find them at medical supply or drug stores that don't sell potato chips.  Use a "dirty" syringe to suck out old fluid the reservoir.  Also a "clean" syringe can be used to reverse bleed the brakes.  I think I tried this on the K1100 and it didn't work, but I always try to push new fluid in from the calipers.

[Elipten]

Can buy on Amazon. Did not help me.  Only pulling and tying handle back overnight solved the issue

[The Mighty Gryphon]

Update, looked at the bike today and noticed some fluid leaking from the master cylinder cover.  Removed the cover and found it full of brake fluid.  Don't know if it is just from working the lever to bleed or it's leaking past the rear seal. 

On the plus side, the lever travel is correct and the feel is firm.  Looks like it doesn't take much air to whack the brake operation.

Will check again tomorrow.  Fingers crossed that I didn't whack the seal from overstroking the lever.  I'm not really up for taking this thing apart and/or replacing the piston.