Problems Rebuilding Front Master Cylinder

[jmd_forest]

I just finished rebuilding the front Master Cylinder on our father/son cafe project (better the bike stops fast than goes fast) and am getting no pressure that I can see. When I pump the handle the brake fluid pumps into the reservoir. The cylinder seemed in reasonable good shape and I polished it up with 360 grit wet/dry paper before assembly. The exploded view in the manual does not match our unit exactly and is also slightly unclear. I believe I've got the rubber seal around the piston on correctly (fat end of the bevel towards the hose) but not so sure about the rubber seal on top of the spring. Does the spring fit into the cup end of the seal or does the cup end of the seal point towards the piston?Any other ideas on why I'm betting no pressure? I do get fluid down to the caliber which has also been rebuilt. The handle is not just soft, there's no resistance at all.

 

[bcware]

Is there any way you can get photos? If you can, pull the internals out and line them up in the order you inserted them. Pictures should inspire more responses.

 

[bentwrench]

No resistance, but you have fluid coming through? Sounds like the piston is doing what it's supposed to, but maybe air in the lines still?

 

[jmd_forest]

Its possible there is still air but I ran thru 3 resivoirs of fluid bleeding it already. I suspect I've screwed the pooch on the MC rebuld. Time to pull it apart and figure out what I did wrong.

 

[OliverB]

It can sometimes be difficult to get all the air out of things and it will behave as you said.

 

[drewpy]

the open end of the rubber "cups" should point towards the hose connection, so it pushes the fluid forward. hope that helps

 

[jmd_forest]

drewpy,

I'm pretty sure I put it in the other way because that's the way I remember it coming out. Its hard to tell from the picture you provided (the same one I used when putting it back together) which way the cup pointed. I guess I'll be pulling it apart this weekend.

 

[jmd_forest]

Pulled apart the MC and I had the rubber cup at the end of the spring in backwards. Flipped it around, reassembled the whole thing and now I'm pumping fluid out the bleeder and the handle get hard. A little more bleeding and it looks like I'll be good to go.

 

[drewpy]

you little bleeder you!

 

[ChrisXS78]

Hey guys,

I'm in the middle of this right now. How did you get the seal that sits on the Piston body over the flared ends of the piston? I don't think it will stretch and I'm worried about ripping the seal.

Cheers,

Chris

 

[drewpy]

put it in hot water from the kettle to loosen it a bit, bit of cardboard into a tube and push over the piston using the cardboard as a feeder.

 

[ChrisXS78]

Thanks Drewpy. Worked like a charm!

Now, I also find this bloody diagram to be a bit shit. The washer goes between the piston end and the seal closest to the hose??

EDIT: Just to update this post for the community at large:

The order of parts is as follows: From closest to hose - Spring, Seal (Cupped towards Hose), Piston with seal mounted on it - again cupped towards hose, washer, snap ring, and the dust seal.

 

[ChrisXS78]

Well, I got it all together and when I attempted to bleed the brakes the MC wasn't pushing any fluid down at all.

I put everything in in the order posted above. I had both seals set up with the flared end facing the hose. I checked the ports in the reservoir before putting it back together and they were clean as a whistle. I cleaned the banjo bolt.

It feels as if the lever isn't grabbing any fluid. It just moves back and forth easily without any feeling of drag or pressure on the lever.

 

[domains]

without fill up cap pump lever 1/4 20-30 times till no more bubbles... stop 20-30 mins and repete this till no more bubbles.

 

[drewpy]

check the splooge hole inside the reservoir, it lets fluid behind the cup expand into the reservoir.

sometimes back filling works and also opening the bleed, then zip tie the closed lever over night to let the air escape through the splooge hole.

it was a bugger for mine to do with twin disks and also my mate's cb750 benefited by the zip tie method

 

[OnePhate]

How did you all get that E clip out?

 

[bentwrench]

Two long nails. Hold with one, press with the other.

 

[XSLeo]

Often it takes a bit of work to bleed just the master cylinder. With the line unhooked put your finger tip over the hole. Now slowly pump the lever. As you pump the lever the air will get pumped out past your finger and your finger won't let any back in. After a few pumps you should get fluid coming out around your finger.
Once you get that farHook the line up. Hold ther line straight up. Hold the end of the line with your thumb and finger. Repeat the bleeding to get fluid to fill the line. Now hook the line to the caliper. It should now bleed at the bledder on the caliper.
If you have two shorter lines with a block on the trees. Bleed each section of line from the top down.
Leo

 

[jmd_forest]

If after pumping awhile while bleeding and it is not pushing any fluid out the end of the line its reasonable to assume that there may be a problem with the Master Cylinder, especially if you just rebuilt it. Also, make sure to keep the cylinder full of fluid while pumping. I had to fill the cup with fluid 3 or 4 times while bleeeding.

 

[drewpy]

If after pumping awhile while bleeding and it is not pushing any fluid out the end of the line its reasonable to assume that there may be a problem with the Master Cylinder, especially if you just rebuilt it. Also, make sure to keep the cylinder full of fluid while pumping. I had to fill the cup with fluid 3 or 4 times while bleeeding.

aye, keep pumping the volume