1987 K100 master cylinder bubbles when brake lever pulled

[Wasserwolf]

Hello,

In the process of changing brake fluid on the front. Got the new fluid in but with every pull of the brake lever bubbles pop up in the reservoir.  I tested the fittings and lines and all are tight. I assume the main seal in the master is bad or loaded  with debris.  Is the right approach to now remove the master to inspect the seals?

[Laitch]

Please describe what you did to change the brake fluid.

[The Mighty Gryphon]

It's not unusual to have bubbles when flushing and changing brake fluid.  This often also involves a spongy lever as well.

Several of the inmates here as well as the writer have had success with a technique that involves tying the lever in at a bit more than the half travel position, turning the bars so the reservoir is as high as possible and with the reservoir cover loosened, leaving the bike to sit overnight.  It also doesn't hurt to gently tap the brake lines occasionally to loosen any bubbles that may be attached to the walls of the lines.

[Wasserwolf]

Laitch

I put a 6mm hose on the nipple. I opened the reservoir and with a syringe extracted as much fluid as possible without getting to the seal. I added fresh fluid and when I cracked the nipple open, I had a friend pull the brake lever slowly back. I did this for the left and then right side until all the bad fluid was expelled. I shut the valve everytime the brake lever reached full travel and before he released it.  I have brake pressure but every pull pushes fluid back into the reservoir  with enough force that it "pops" the fluid . I have a video , but its  the wrong format to post here. I looked and it again in slow motion and can see a"wave" in the fluid as I pull the lever.

[Martin]

They can be a pain to bleed and sometimes you have to leave them overnight as per Gryphs post. I had a similar problem which was caused by a tear in the lining of the OEM rubber brake hose. The tear formed a rubber flap that formed a one way valve allowing air to be drawn in without letting any fluid leak out. I had a spongy lever action. I found it by blocking off the hose at the caliper and it was still spongy.  I then blocked off near the master cylinder and the sponginess disappeared. I used to have a hydraulic specialist across the road from me who said that he had seen this happen before.
Regards Martin.

[Wasserwolf]

I took a close look at the reservoir when I pull on the lever. It is not air coming back up through, it is a pressure wave coming thorough the fluid. I have a friend who is a master mechanic and he thinks I have a piston sticking in one of the calipers. This makes sense to me because the harder/faster I pull on the lever, the larger the wave through the fluid. I guess, I need to pull them apart. So much for a quick fix.

[Laitch]

I have a video , but its  the wrong format to post here. I looked and it again in slow motion and can see a"wave" in the fluid as I pull the lever.

Generally, we post videos on YouTube then post the links to the videos here but most of us have seen bubbles in brake fluid so it probably wouldn't be worth the effort.

If this were my concern, I'd fill the reservoir, fasten the reservoir cap with the screws, bleed the caliper/s at the nipple/s until there were no bubbles being expelled from the bleed tube. If the brakes felt firm and the reservoir fluid was at an acceptable lever, I'd be finished with the job. If the brakes were firm, the brake lever not moving an excessive distance and the reservoir needed topping, I'd remove the cap, top it up then fasten the cap; I wouldn't squeeze the lever to look for waves or bubbles.  If the brakes were not firm, I'd try the alternative methods that Gryph and Martin are recommending.

We're all doomed anyway so don't worry about it.

[Wasserwolf]

I took off both calibers and removed the cover. On one side the spring holding the pad pins had wedged itself to one side and was interfering with the movement of the piston. I reset it to center and no more issues. Thanks guys.

[johnny]

you pulling the lever too fast... sloe it down...

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[Wasserwolf]

you pulling the lever too fast... slow it down...

I agree, I was kind of aggressive on the pull. Thanks!

[The Mighty Gryphon]

They also don't like to be pulled to the grip.  I lay a loop of 1/2" rope over the grip to act as a stop.  I just pull until it touches.  I do that so I can concentrate on the bleeder.

Have to be careful of the rear master cylinder as well.