Revvy Kevvy
XS400 Enthusiast
On my abandoned XS400 I opened the brake master up some months ago and it was in terrible condition. Ordered some rebuild kits and a couple of master cylinder hones and went off for a buisiness trip. Came back and one night rebuilt the master brake cylinder put it back together and not having any brake fluid on hand just left it be.
This week filled it with brake fluid and pumped it, absolutely nothing. I figured I must have rebuilt it wrong so I checked and as far as I can tell I have put all the rubbers in the correct orientation. The picture shows the orientation.
But, looking at how it works I don't see how it can work even if I assembled it right. The hole for the reservoir comes out between the two seals on the piston. So any fluid is going to just sit between the two rubber seals and go back and forth. I don't see how it can pump the fluid. For it to pump the fluid it would have to put the fluid at the front of the piston not between the two pistons. All my measurements and doing my best to look in the hole seem to tell me that that is exactly what is going on.
Does it mean I have really screwed up the rebuild? did I orientate something wrong? or was it rebuilt 25 years ago by somebody other than me before the start of the millennial and I am getting confused by a wrongly rebuilt master cylinder...because if its rebuilt correct I dont see how it would work, how would it pump the fluid down the brake line to the brake piston if the fluid is between the two seals on the piston....it has to be in front of the piston to be pushed.
If its correct, the hole for the fluid will be between the front rubber seal and the seal on the piston, this means the fluid will just move between the two seals and cannot get to the front of the piston and go down the brake line to the brake piston. This idea would never work and when I press the master piston in and out what I described is exactly what is happening...no brake fluid comes out the front of the master cylinder but I can see the piston moving back and forth in the hole.
This week filled it with brake fluid and pumped it, absolutely nothing. I figured I must have rebuilt it wrong so I checked and as far as I can tell I have put all the rubbers in the correct orientation. The picture shows the orientation.
But, looking at how it works I don't see how it can work even if I assembled it right. The hole for the reservoir comes out between the two seals on the piston. So any fluid is going to just sit between the two rubber seals and go back and forth. I don't see how it can pump the fluid. For it to pump the fluid it would have to put the fluid at the front of the piston not between the two pistons. All my measurements and doing my best to look in the hole seem to tell me that that is exactly what is going on.
Does it mean I have really screwed up the rebuild? did I orientate something wrong? or was it rebuilt 25 years ago by somebody other than me before the start of the millennial and I am getting confused by a wrongly rebuilt master cylinder...because if its rebuilt correct I dont see how it would work, how would it pump the fluid down the brake line to the brake piston if the fluid is between the two seals on the piston....it has to be in front of the piston to be pushed.
If its correct, the hole for the fluid will be between the front rubber seal and the seal on the piston, this means the fluid will just move between the two seals and cannot get to the front of the piston and go down the brake line to the brake piston. This idea would never work and when I press the master piston in and out what I described is exactly what is happening...no brake fluid comes out the front of the master cylinder but I can see the piston moving back and forth in the hole.
