Rear master cylinder stuck

[Guest]

I am trying to take the rear MC apart from my '79 GS850. Got the cerclip out but the piston is stuck in there. Any ideas on how to get it out? What could I soak it in to dissolve the dried up brake fluid and free up the piston?

thanks in advance
Brian

[mnferwerda]

Douse it with something like PB Blaster, tap it a few times, and then let it sit for awhile (20 minutes or longer). Vinegar will also work. If it doesn't loosen up, do it again. Eventually it will come loose. I had one stuck and it took a few days.

[Thunderpipes]

Hi Brian,I also have a gs750db from 1977. Great bikes with bomb proof engines. I had the same problem with mine. I soaked the whole master cylinder in a bath of hot water and a generous amount of soap powder.leave to soak until the water is cold and use an old toothbrush to remove the crud. Rinse way with clean hot water.If your on lucky like me,the piston will move free. I hope this helps,let me know how you get on.

[canuckxxx]

Thunderpipes, mnferwerda, thanks for the thoughtful responses. I got the MC apart by pushing through the other end, where the banjo bolt goes. I used the non-cutting end of a drill bit that just fit through the hole and it pushed it into the MC against the workbench. It took very little force and as soon as it moved a little the piston fell out. Before that I had soaked the push end of the MC with brake fluid, brake cleaner and the last was a mixture of ATF and acetone. So maybe one of those loosened it up.

I thought about boiling the whole MC in water but never made it to that stage.

[Nessism]

Heat works well to break down varnish. Be sure to check the bore for pitting after you get the piston out. It's extremely common for there to be pitting in the metal which precludes rebuilding.

[tobaccorancher]

I have a front master cylinder that I want to re-build, but cannot get the piston out using soaking, heat, ....I saw a reference on youtube where a guy used a grease gun and hydraulic pressure to remove brake pistons. Has anyone ever tried that? All it would take is an extra master cylinder and a short brake line with banjo fittings on both sides....?

[tobaccorancher]

OK, I answered my own question. A grease gun can create 15,000 psi of pressure while a master cylinder creates much less pressure. So, I think I will look for a zirc that will fit and try that.

[rphillips]

Don't think I've seen zirc with that big of threads, possibly easier to get the threaded end of the grease gun tube (hose) adapted to the master cylinder outlet.

[tobaccorancher]

Sounds like a plan. I'll start looking for fittings.