I have a Mazda b 2200 1991, 4 cyl, 2 door

The problem with this pick-up is that is throwing oil out the valve cover, I replace the piston rings and cleaned everything, but it still doing it, it looks like if there was presure building up in the valve cover, it threw out almost all the oil the last time, after replacing the piston rings with new ones, it does it, but less. Someone told me that it could be the engine block, but I still think its the valves that are not closing rigth, but would like someone to help me, dont want to spend money if I dont have to. Thanks and I apreciate your help. :D