Bunko Front Master Cylinder

[jonr]

OK Gang, I am panic stricken here. Rally is on Friday and I've got a front master cylinder that won't pump any fluid in my front lines.

Here's the scoop on my 78 GS750:

Stock rubber brake lines showing heavy cracking. Got to replace these things before I take the 800+ mile roadtrip to Galena. I carefully remove all the old lines and clean out the old master cylinder. There is tons of rusty residue inside the front brakeline system becasue of a metal plate that helps secure the reservoir to the lever housing. I clean out the reservoir and put a new stainless plate in. I tighten the screws and and replace the lines with Goodridge SS lines. All is secure and I go to bleed the brakes and NO fluid will pump into the lines. The fluid level never drops in the reservoir and the lever is squishy all the time. The only thing I can imagine is that some of the residue got down into the plunger mechanism inside the master cylinder and is blocking fluid movement. Can anybody think of anything physical that would prohibit fluid from entering the system?

Sorry for the long post.

-Jon

 

[Mr. Jiggles]

You may need to prime the master cylinder. Disconnect the banjo bolt from the MC, fill the MC with fluid if you haven't already, hold your thumb firmly over the MC outlet and pump the brake lever. After a couple of pumps you should feel pressure behind your thumb with fluid forcing its way out. That should hopefully do the trick.

Jeff

 

[jonr]

Jeff Jiggles:

Thanks for the recommendation. I'll give it a try after I reassemble the MC. I got all remaining gunk out and will reassemble after I get a new retaining ring. The old one is quite rusty.

Keep your fingers crossed!
jon 8O

 

[bwringer]

The Jiggler is correct -- you have to prime any MC upon replacement. On cars, you have to stick the new MC on a bench, cover the outlets, and pump the piston with a screwdriver to get all the air out of the MC, or at least enough so it'll start pumping brake fluid.

If the rust was causing a sealing problem, it would probably leak rather than just not working.

If you need a rebuild kit fast, there's a good chance that Cycle Recycle II up in Indy would have it.

 

[jonr]

Jeff and BW,

So far so good. I am at my workshop and have successfully reassembled the MC. Made a custom stainless washer and got a new retaining clip since the old one was literally rusted halfway through. Upon first bench bleeding, I found that it I squeezed the lever and then stuck my finger over the brake line opening that it quite instantly sucked fluid down into the piston. Another squeeze with my finger over the hole and brake fluid came out under as high of pressure as my finger could maintain.

I'm hoping that it is this easy when I get back to the bike in the morning. I've GOT TO make this Galena rally. Would be crushed if I didn't go because of bleeding the brakes! I'll report back once I get it installed.

Thanks to you both for the suggestions.
-jon :lol: