Donato,

Your shaft brake cylinder looks a little bit different than others I have seen on a Super Maramu, but I assume  it works the same way.

I have attached below a drawing from the original manufacturer (who still makes these, or at least was able to supply me with parts, as of two years ago.) 

In that center hole there should be a bolt that is used to adjust the amount of compression on the spring that clamps the pads closed on the disk when the engine is not running. The back side of the cylinder (where that bolt goes in) should never have oil in it.

When the engine starts, the transmission oil pump generates pressure that push down on top of the piston, compressing the spring, and releasing the disk from the pressure of the pads.

If there is oil coming out the back end of the cylinder, then the seal between the piston and the inside of the cylinder is compromised.  The unit will need to be disassembled to replace that seal.

If you decide to disassemble it yourself, be very careful!  That large spring inside is under a lot of pressure.

Bill Kinney
SM160, Harmonie
Port Louis, Grenada
http://www.cruisingconsulting.com