The hpfp is filled with hydraulic fluid (CHF11S). This can leak out over time, and cause the pump to not function properly. This was the case with my pump. You can measure the depth of the diaphragm, I believe it should not exceed 20mm to the outer rim. I went from 27mm to 16mm)
I removed the pump, opened the pump, removed the cam gear from the shaft, and removed the rotary seal on the shaft. Then reassembled, without re-installing the oil seal. You then fill the pump with oil via the shaft. You will need to cycled the pump, with a drill or impact in-between filling it, to make sure all the air is out. Once no more bubbles appear, install new oil seal, thread on cam gear, and you should be good to go.
I'm afraid I only took a few pictures while doing this.
Be aware, that this is just one of a few possible reasons that your hpfp may fail. If you open the pump, and it is filled with fuel/oil mix, there are o-rings that you will need to replace. A special tool is required to remove these cylinders to get to the o-ring. If one or more of the bellows have split, you will have to replace the pump.
I have driven about 1500km so far on the rebuilt pump, and no issues. My recent logs also show that the pump is performing as it should.
Remove these 3 torx bolts:
Clamp cam gear in vice:
Put screwdriver in this slot, and hit with hammer - counter-clockwise - to loosen cam gear on shaft
Then remove the cam, and remove the shaft and oil seal. Then reassemble without installing your new oil seal yet.
Once reassembled, apply a small amount of vacuum to the diaphragm - I inserted a tube and then sucked on it, then clamped the tube to hold the vacuum (this allows a little more oil into the pump, and will ensure good internal oil pressure)
Fill through this gap. You can see I threaded a bolt into the shaft, where the cam goes, to enable me to cycle the pump with my drill. Fill with CHF11S, and cycle the pump, fill, cycle, etc etc - It took about 45mins for me to get no more bubbles. Once the bubbles have stopped, remove the bolt, slide on your new seal, and make sure it is seated correctly. Then thread on the cam, remove your vacuum tube, and you should be golden. No need to tighten the cam, the rotation of the engine will do that for you.
Oil seal:
I'm just sharing my experience that worked for me. Good luck!
