Master Cylinder

[JBwell]

1980 r65 , has the master cylinder on handle bar.  Looking down into the barrel, there are two holes.  One about center and one offset.  The offset hole is completely clear but the center hole is not.  Actually looking through the cylinder part I can see where the offset hole comes out, but there is no such opening for the center hole.  Is that center hole supposed to be clear just like the offset hole or is it really small like in a carb jet?  Thanks all - JON

[Barry]

I might have the holes mixed up as this is from memory. The offset hole I think is the vent hole. That's the one that spurts when you release the lever. It's this hole that the system fills through and fluid returns from when the lever is released. The centre hole is what they call the compensation hole as it communicates with the gap between the two piston seals. This hole should be open but you wouldn't be able to poke something through it very far before hitting the piston shank.  You shouldn't observe any flow through the compensation port if the seals are good.

In this diagram what I'm calling the compensation port they have called the replenishing port which I think is very misleading because it's actually the vent port that replenishes when the things are working correctly. Hope that's not too confusing but it's the best diagram I can find to explain how the master cylinder works.

When the brake is applied the piston move from right to left on the diagram. If the primary piston seal should pass some fluid instead of the fluid leaking out of the master cylinder assembly it returns to the reservoir via the compensation port. It means in practice that when the primary seal goes bad you can still pump the brakes to get some stopping power and the fluid just keeps circulating back into the reservoir. Without the compensation port you wouldn't pump for long before eventually running out of fluid. So that would be a good reason to ensure the compensation port is not blocked.  It's easy to see why you can't poke something very far through the centre port and in practice the centre shank on our piston is thicker than shown in the diagram.