It is quite possible the front cover is 'warped' a bit and not evenly compressing the O-ring. Try checking the flatness with a sheet of glass just barely wet with oil-seeing if theoil film is transferred uniformly to the (dry) pump cover. It it is not, you can 'flatten' the cover by using a sheet of 240-400 grit Silicon carbide abrasive paper on a sheet of glass,using light oil (WD-40) as a grinding lubricant. Rub the cover back and forth in a straight line, stopping and rotating it 90 degrees every few strokes to ensure even metal removal. Recheck flatness , repeat until it is flat.

You can also put a few layers of teflon tape under the 'O-ring' to increase the compression placed on it. Remember to seal the bolt threads.