If it's still dieseling you must have grease in front of it.
How well does the seal fit is it tight or loose?
How badly is it dieseling, will a dozen or so shots clear it?
Failing that you will have to strip, clean and reassemble
I found just putting the grease in the groove round the outside of the seal, avoiding the front edge altogether, does the trick. The piston moving up and down should spread it out.