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View Full Version : K100 Master Cylinder Bleeding?



gstom
04-07-2004, 07:35 PM
I replaced the front brake pads on my '88 K100LT. (first time I ever did it) I bled the brakes using a "mighty-Vac" hand vacuum pump which I have used sucessfully several times in bleeding car brakes.
I have good pressure at the brake lever, but the master cylinder "gurgles" and spits back into the reservoir when I pull the lever. Further bleeding failed to relieve this condition. Now the brake reservoir leaks from fluid escaping from around the screws that hold the lid on. What can I do to fix this annoying (and messy) problem? Is a rebuild of the master cylinder needed? :dunno

kbasa
04-07-2004, 10:24 PM
Mine has always spit back into the reservoir, usually when I let go of the lever (if my brain is working correctly here).

Do you have too much fluid in the reservoir? It doesn't get filled all the way to the top.

90288
04-08-2004, 02:22 AM
I've done this gig several times using the Mighty Vac method and I'm always amazed at the amount of fluid I have to go through in order to get a solid feeling brake and no air bubbles.

DO NOT underestimate how much fluid you'll have to go through.

I recall going through almost 2 little bottles before all the crap and air bubbles were flushed out. Make sure your connections are tight as well otherwise you're just bleeding air back into the system.

You might want to flush it and then wait a few days or weeks and bleed her again. (I'm not even go into making sure you're brakes are working properly discussion before riding.)

Good Luck!

MANICMECHANIC
04-08-2004, 05:08 AM
One technique I use is to take the bleed hose and the collection cup and run it high, as in over the top of the fender. The idea is that you'll see air bubbles, but you'll also have a fluid level on top of the bleeder, precluding air from returning. Crack the bleeder, then pump the lever, keeping the reservoir full. Once you see the bleed line is clear of air and old fluid, shut the bleeder and top off the reservoir. I usually don't use a vacuum any more.

BRAHMA
04-11-2004, 03:24 AM
Make sure the connection under the master cylinder where it goes into the handlebar is really sealed. I just bled my brakes two days ago to meet the annual change spec with my mighty-vac and no bubbles came up through my reservoir. I have had to change my reservoir seal before- I think I recall it was jst an o-ring.

deilenberger
04-13-2004, 01:49 AM
Originally posted by manic mechanic
One technique I use is to take the bleed hose and the collection cup and run it high, as in over the top of the fender. The idea is that you'll see air bubbles, but you'll also have a fluid level on top of the bleeder, precluding air from returning. Crack the bleeder, then pump the lever, keeping the reservoir full. Once you see the bleed line is clear of air and old fluid, shut the bleeder and top off the reservoir. I usually don't use a vacuum any more.

OUCH! I'm distressed when I hear about people pumping the lever to bleed or flush brakes. This old-time technique will usually result in needing a new master cylinder in a rather short period of time, unless you were REALLY religious about yearly fluid flushes.

I thought we'd discussed using a power-bleeder to flush/bleed the brakes on K bikes, but in case we haven't:

1. It costs about $10 and a bit of begging to make

2. It won't destroy your master cylinder

3. You can bleed (ie - remove the air) from a completely dry K ABS-1 system in about 5 minutes (which is a long and circuitous path).

4. There is no mistaking air getting sucked in around the nipple for air in the system (my big gripe with Mitivac, aside from the mess it makes)

To make one: http://www.apexcone.com/JimPowellHomepage/Bleeder/bleeder.html

Then go beg a cap off a bad master cylinder from your friendly local dealer.

Works a charm. Once you use one - you'll never go back to hand-bleeding.

deilenberger
04-13-2004, 03:44 AM
Originally posted by GS Tom
I replaced the front brake pads on my '88 K100LT. (first time I ever did it) I bled the brakes using a "mighty-Vac" hand vacuum pump which I have used sucessfully several times in bleeding car brakes.
I have good pressure at the brake lever, but the master cylinder "gurgles" and spits back into the reservoir when I pull the lever. Further bleeding failed to relieve this condition. Now the brake reservoir leaks from fluid escaping from around the screws that hold the lid on. What can I do to fix this annoying (and messy) problem? Is a rebuild of the master cylinder needed? :dunno

If what you describe as a gurgle is just a little spit of fluid back into the master cylinder reservoir - that's absolutely normal. If it didn't happen your brakes would lock on (it's the relief passage that allows fluid back into the reservoir when the brake is released).

If it's air bubbles - then you need more bleeding. See my other post on using a power-bleeder...

The fluid around the screws is probably some fluid that got above the bellows gasket/seal on the cap. You should remove and dry that seal with clean lint-free cloth (that you then throw away).

When the screws are tightened and the gasket/seal is correctly in place - the screws are isolated from the fluid in the reservoir. If it continues - see a dealer for a new cap gasket. The gasket should be installed so the center section (which is a bellows of sort - to allow the fluid level to drop with pad wear and not allow air to reach the fluid) - is fully retracted towards the cap.

Meanwhile - it's usually not necessary to "bleed" the brakes after changing pads... you aren't supposed to open the system. The capacity of the reservoir is such that it shouldn't need fluid added between pad changes. It will drop - but shouldn't drop below the LOW mark on it - as the pads wear. When you replace the pads - the fluid level should be back at the full mark.

What you do want to do is "flush" the system every year - which is much easier to do with a power bleeder since the reservoir never goes dry, and you can easily do it without any help. The flush involves passing new fluid through the system until all of the old fluid is out of the system. If done correctly - no air should be introduced into the system while doing it.

Best, and HTH..