Brake problem...........

RPV is fitted and definitely improves the brakes! ..BUT.........before the RPV was fitted and now,after its fitted I still have fluid spluttering out of the breather hole in the cap cos the plastic disc,valve thing in the cap broke off,so I fitted a union and a plastic pipe about 12" long mounted vertically of f the union. Im still getting fluid going up the pipe after about half hours run(ie various times applying the brakes) I'm not convinced that the vent valve that broke would stop the amount of pressure needed to push the fluid up the plastic pipe Ive fitted.I have the fluid at the correct level in the m/cyl . The union I fitted in the cap sticks down around 4 or 5 mm into the cap,and I suppose fluid could touch this as it is washed around inside the m/cyl.(I suppose it must do ,as how else would the fluid get in the pipe? )There seems to be gaps of air/fluid/air etc in the pipe,so could it be that the fluid is going up as mentioned,then being sucked down as the level in the m/cyl rises and lowers as the brakes are applied?I ve looked at other caps,and some dont have a valve,just a hole. Im also told that some more modern caps dont have a hole at all.Surely this wont allow air in ,therefore causing a vacuum on top of the fluid in the m/cyl,which would hold the fluid back when braking? Im going to have to buy a complete new m/cylinder and cap with a vent valve in it,as I cant seem to buy a cap with the plastic vent in it on its own.