Front Master Cylinder Problem on a '79 CB650

S

Strife

New Member
I just got to the point where I am installing everything on the bars, so the master cylinder, switches, etc, and they master cylinder isn't working. When we first started working with it, we got a cloud of rust that came up when we put in brake fluid. We took it apart and cleaned it with CLR, which seemed to clear everything out. I put it on the bike and hooked it up the the caliper, and was able to bleed the system just fine with a little bleeder kit I had. We got the fluid running, pumping the brakes worked great, and we worked out all of the air bubbles. One odd thing was when you pumped the brakes, the fluid would go into the bleeder kits' little container like normal, but releasing the brake would suck the fluid back in. Is that normal (I'm a newbie lol)?

Anyways, We ended up having to disconnect the master caliper because we ran the hose wrong, which meant bleeding the brakes again since air would have been reintroduced into the system. Now, the master cylinder doesn't works again. It's pushing out almost no fluid. Any ideas as to what is going on? I do have clubman handlebars, rather then stockers, does that make a difference? Also, here is a link to the specific "aftermarket" master cylinder that I bought: http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=120458284065&_trksid=p2759.l1259.

What do you guys think?
 
So the maker of the aftermarket part sent me a replacement part for free, which was really nice of him. I installed the new one, and everything on the master cylinder seems to work fine. I was able to bleed out the brake system just fine. But now if I plug the bleeder nipple and try to engage the brakes, the cap just pops off. If I hold that cap on and try to apply the brakes, the lever won't budge. So it seems that my master caliper is completely frozen. How do I go about fixing this? Do I just need ot pony up and by a replacement caliper? Should I take it in? Or is there something I can do?
 
Distinguish between "brake caliper" and "master cylinder" (lever).

If you've got a new master cylinder, and fluid coming out of the caliper bleeder, then you've got fluid moving, which is good.

Now your caliper pistons could be rusted up or frozen which would have them stuck in your caliper. You could remove them with plies if you can get a solid grip on them (without pads), or take them to a shop and say "stick an air needle into my bleed screw hole" and that'll pop them out...probably. If they don't move out, try moving them in with grease on the exposed area of piston.
 
Yea I'm referring to the caliper, aka the actual brakes on the front tire. The cylinder (lever and reservoir) is working well. I tried using CLR on the caliper, several times, nothing budged. I can try taking the caliper in and doing the air needle trick you mentioned. Do you need any sort of special attachment to do that? I have a air compressor and hose here at the house, can I use that to try and pop out the caliper?
 
When you remove the caliper, you'll be left with a hole where the line bung went into. You need to force air into that hole in order to push the piston out. Whether you use an air needle or just any old air nozzle with trigger, I'd be extremely careful that you don't send the piston flying across the shop...or worse yet into your face.

If water has got into or around the piston it'll corrode it so much that it won't come out with pressure of the master cylinder, and more then likely it will not budge until exploding apart when you hit it with air.

I rebuilt a whole set of RX7 brakes this way and they worked awesome after greasing up all the pistons.
 
Its not often that you need to replace the whole caliper unless the inside of the piston cylinder is in really bad shape. All that is required is that you strip the caliper of everything and give it a good cleaning. Pay special attention to the piston seal groove. Remove the rubber seal and clean out the channel. I used a small brass wire dremel wheel to scrape out all the crud and corrosion. You might surprise yourself to see how much stuff comes out of there. Use the wheel to dress the rest of the piston cylinder if corrosion is present. Clean the ports as well. If the piston moves freely inside the shaft, without the seal, then you are in good shape. Check the condition of the piston as well. I have found that a little pitting is ok but anything excessive will hinder the tight seal and back/forth operability.

FYI: It is common for corrosion to form behind the piston seal in the seal groove. When that happens, the corrosion pushes the seal out, putting excessive pressure against the piston. This will cause the piston to seize or move very slowly.
 
There is more force put upon the piston to engage then disengage so to answer your question, one can say yes. An uneven piston surface would cause undue friction on the piston/seal causing it to possible hang up upon retraction. The pitting would cause both a seal issue and friction problem. If there is any question, just replace it. Brakes are one thing that should not be skimped on.
 
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