Suzuki 2 Strokes

Hi, have just replaced the oil line check valve to the left cyl with new one as the old one was shot and still blowing back through new one. 2 were supplied and I have tried both. Just can`t understand it. Checked the right corresponding one with the engine running and it seems to have about the same pressure. The oil lines come out of the pump one for left and one right, and then split into 2, one supplying the barrel and one the crank bearing on either side. The faulty left crank one or lower one blows back and even disrupts the higher line via their junction. You can clearly see oil being blown back through the line. What are chances both new ones are faulty? How much pressure would it take to wreck the old one and then blow by the new one(s). What inside the engine could cause this pressure to be greater than normal? The bike has been slowly restored and new top end, good compression, just always had a slight misfire on the right side, after a period of riding, maybe 5 mins maybe 15 mins it suddenly starts to misfire real bad and then becomes unrideable. I have chased this through all the systems, (rebuilt carbs, replaced outer crank seals, rebuilt oil pump seals, coils, plugs, plug leads, caps, points, condensers, Checked and re-checked the timing etc) now awaiting old original rectifier and regulator to replace the modern combined rect/reg wired into the bike when bought. Any ideas? Steve.