O-ring in hydraulic self-levelling pump
 

I have been experiencing an oil consumption problem with my 190E 2.3-16 since I bought it 2 years ago. It has 46,000 miles so I didn't think it was a wear related problem. I recently replaced the valve cover gasket as it appears the oil is escaping high on the engine. I pulled the hydraulic pump off to replace the o-ring but when I tried to replace the pump, the o-ring bunched up on the top of the seating area where the valve cover seats. I tried a few times to fix this but ended up putting the old one back on. I used a gasket dressing for all surfaces, and have not seen a drop since, but am wondering how to replace the o-ring how to get the fit right.