# Explain how this motorcycle master cylinder diagram is supposed to work. **URL:** \ **Category:** Factual Questions\ **Created:** [October 1, 2017, 8:20am UTC](https://boards.straightdope.com/t/explain-how-this-motorcycle-master-cylinder-diagram-is-supposed-to-work/797708 "2017-10-01T08:20:41Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![Clarence01](https://avatars.discourse-cdn.com/v4/letter/c/d2c977/32.png) [@Clarence01](https://boards.straightdope.com/u/Clarence01)\ **Post date:** [October 1, 2017, 8:20am UTC](https://boards.straightdope.com/t/explain-how-this-motorcycle-master-cylinder-diagram-is-supposed-to-work/797708/1 "2017-10-01T08:20:41Z")
Photo: [http://tinypic.com/r/1zwgm61/9](http://tinypic.com/r/1zwgm61/9) Hi, I rebuilt the rear master cylinder and now the brake fluid is not passing through (you can see the fluid sitting in there in the unit)… so I’m seeking a greater understanding of how this is supposed to work because the supply port [H] is not clogged. The fluid needs to pass through the top part of the cylinder where the spring [A] is housed to reach the rest of the brake line but how is it going to do that when the primary cup [C] which sits flush against the inner wall and thereby blocks the fluid never descends lower than that supply port? If you need a hint on how this is supposed to work… The diagram shows how things should look without any brake pressure applied. With the brake pressed the piston \*\* gets pushed up which compresses the spring [A]. The primary cup’s [C] position would be roughly where the line to the right of label F is. Things would seemingly make a lot more sense if the ports H and G were interchanged on the diagram but that’s not how it says that it should be… not that I could fix the part anyways. Thanks in advance for any help in resolving my issue here! ---
**Author:** ![somepumpkin](https://avatars.discourse-cdn.com/v4/letter/s/7feea3/32.png) [@somepumpkin](https://boards.straightdope.com/u/somepumpkin)\ **Post date:** [October 1, 2017, 12:55pm UTC](https://boards.straightdope.com/t/explain-how-this-motorcycle-master-cylinder-diagram-is-supposed-to-work/797708/2 "2017-10-01T12:55:07Z")
I believe there should be a small hole in one of those two dimples (not in the center). It’s terribly small. I used a section of safety wire to clean the one on my BMW R100. ---
**Author:** ![Clarence01](https://avatars.discourse-cdn.com/v4/letter/c/d2c977/32.png) [@Clarence01](https://boards.straightdope.com/u/Clarence01)\ **Post date:** [October 5, 2017, 6:21pm UTC](https://boards.straightdope.com/t/explain-how-this-motorcycle-master-cylinder-diagram-is-supposed-to-work/797708/3 "2017-10-05T18:21:48Z")
> [@somepumpkin](#): > > I believe there should be a small hole in one of those two dimples (not in the center). It’s terribly small. I used a section of safety wire to clean the one on my BMW R100. It didn’t occur to me that such a tiny hole (in the relief port) could affect the system the way I described but in case it is, I’ve cleaned it with a pin and plan to test the system again today. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [October 5, 2017, 9:14pm UTC](https://boards.straightdope.com/t/explain-how-this-motorcycle-master-cylinder-diagram-is-supposed-to-work/797708/4 "2017-10-05T21:14:54Z")
> [@Clarence01](#): > > Photo: [http://tinypic.com/r/1zwgm61/9](http://tinypic.com/r/1zwgm61/9) > > Hi, I rebuilt the rear master cylinder and now the brake fluid is not passing through (you can see the fluid sitting in there in the unit)… so I’m seeking a greater understanding of how this is supposed to work because the supply port [H] is not clogged. The fluid needs to pass through the top part of the cylinder where the spring [A] is housed to reach the rest of the brake line but how is it going to do that when the primary cup [C] which sits flush against the inner wall and thereby blocks the fluid never descends lower than that supply port? > > If you need a hint on how this is supposed to work… The diagram shows how things should look without any brake pressure applied. With the brake pressed the piston \*\* gets pushed up which compresses the spring [A]. The primary cup’s [C] position would be roughly where the line to the right of label F is. Things would seemingly make a lot more sense if the ports H and G were interchanged on the diagram but that’s not how it says that it should be… not that I could fix the part anyways. > > Thanks in advance for any help in resolving my issue here! Did you bleed the MC on the bench before installing? It won’t pass through until bled.