best case scenario, you simply need to bleed your brakes. buy a couple of bottles of fresh brake oil. get a friend. and bleed the brakes. methods are available on net and on this website
worst case scenario
your brake cylinder / seal is about to fail... Replace with NEW immediately.. never compromise on brakes.
things to check.. individually inspect each wheel. do you see any traces of oil leakage? what is the brake oil level? is it constant or does it keep reducing?
if all the wheels are ok.. then dismantle the master cylinder. take of the snap lock ring with a nose plier. take out the pistons.
clean the cylinder thoroughly with soapy water. dry and inspect for any signs of pitting/corrosion. if you observer even a little. than buy brand new master cylinder. do not experiment with used stuff..
if no pitting or erosion, then buy NEW master cylinder repair kit. it will have new pistons, rings seals etc. when installing apply little brake oil to the master cylinder and the seals you are fitting in it.
fit every thing back.. open the bleed valves one tyre at a time and pour in fresh oil at the top... pump the brake pedal very gently and only a little. not fully down.
when you observe fresh oil coming from the bleeding valve then close the valve. repeat for each tyre and keep topping up with fresh oil at the top..