Front disc brake master cylinder

Hello.
When I pump with the lever (without the cap), the oil gushes out of the reservoir like a fountain from the return hole.
Why?

That is because the piston hasn't traveled enough to cover the return port and so the fluid being moved goes out the return port, when the piston then seals off the return port that is when the fluid actually moves the caliper pistons, if you can reduce the excess travel down to a minimum you will get better brakes. The RGM sleeve kit makes this easy, it has an adjustable piston pusher between the lever and piston. Not so easy to acheive with the 13mm bore AN m/c, you have to make a spacer sized to reduce the travel.

But don't adjust so the return port is not used as its a pressure release mechanism.

pierodn said:

Why?

Click to expand...

Because when you release the lever, the caliper seals pull the pistons back from the disc (as under normal circumstances they deform, the pistons only pass through the seals as the pads wear) so the excess fluid needs to vent back to the reservoir. Simple answer is don't pump the lever with the cap (and diapragm) not fitted.