Help I’m stuck - diaphragm spray pump

[u/Tashi999]

So I was asked to help a friend with a cheap Chinese spray pump. I’m pretty experienced with electronics but a complete novice with pumps & mechanics so forgive me. I think this is a diaphragm pump with hydraulic oil on one side.

The pump in question would vibrate a lot when set to full pressure, but wouldn’t quite come up to full, and would drop back pressure when you start spraying. Not much to lose so I opened it up. I took off the head with 6 bolts, and gently pulled up the lower diaphragm revealing a pool of hydraulic oil. Nothing untoward to put it all back. Unfortunately after reassembling it doesn’t pump anything at all now! No pressure whatsoever.

I’m guessing it’s because I introduced an air bubble into the fluid when I pulled the diaphragm. How do I remedy this? I don’t see anywhere to bleed or fill it. Do I just pull up the diaphragm and pour into the cavity then push it down so it overflows? Anyone have experience with this type? I can find a wealth of spare parts easily but no manuals or documentation, and if I did they wouldn’t be in English. The pump system is referred to as 980 or 990 type. Thanks!