Topic: 78F front master brake cylinder rebuild problems

[strawhouse]

So Ive got a 78F 750 front master brake cylinder that I have "tried" to rebuild.  I got a kit for the master and followed exactley what I pulled out I put back in.  Well it doesnt work, when its all installed it doesnt move any fluid thru the master.  I looked at the honda online manual and followed what they did... no luck.  Then I started switching things around, flip a rubber gasket the other way etc... no luck.  It is for the F model because it doesnt fit the K master that Ive got sitting on the shelf, this plunger is too wide.  Everything fits together great and feels right when I put the lever on it but it doesnt pump fluid.  I have tried pumping fluid with the cap on, cap off, on the bike, in my hand, on the vise.... no luck.  I have pulled it apart probably 10 times and I have checked for cracks and I cant see a thing.  The sad part was it worked fine before I "rebuilt" it over the winter, and to top it off I cant find the old parts that I pulled out.  Any pointers?  Any help?

 

[Kevin400F]

If you are certain that all the parts are now installed correctly, you probably just have an air bubble in the master cylinder bore and it needs to be "primed".  Position the MC with the outlet pointing  upward and put your thumb loosely over the outlet port.  Squeeze the lever all the way, then cap the outlet tightly with your thumb, preventing any expelled air from being sucked back into the MC bore.  Repeat until the air is out and you build fluid pressure.  You may have to top up the reservior part way through the process.

Kevin

[strawhouse]

Never thought about that but it makes sense.  I will give that a shot tomorrow and post the results.

[scottly]

I had a heck of a time getting a dry K7 MC bled recently; once it got some fluid into it, by rapidly pumping the lever and letting it self bleed in between pumping sessions, it began to bleed normally. It did have me worried for a while at the time.

[strawhouse]

So I got it to work and it only took about 10 minutes.  I filled up the resevoir and put the cap on, then holding the master in my hand with the out port ( where the brake line connects ) straight up I pushed in the brake handle.  When the handle was all the way in I put my thumb over the out port and let go of the handle.  Then repeated 1 or 2 more times without letting my thumb off of the end until fluid started spraying past my thumb, presto! it was ready to go!

[Kevin400F]

Glad to hear it!