You probably need to remove the piston in the top cover and clean and in stall new piston seals. Make sure the piston bore is good with no corrosion or pitting, also clean and reseal the valve body and make sure the valve is not sticking. I have done a couple of mine with good results
Can someone please tell me what the thread size and type is for the hole in the unloading valve plug. I have a 1962 super major and am having difficulty removing the unloading valve plug. I have tried a 3/8" UNF bolt but it binds after about 1 turn and will damage the thread if I force it.