@Geoffduk ’s post reminded me of an issue I had with bleeding brakes, despite having fitted brand new bleed nipples. At the point of near insanity, I realised that the new nipples, despite having been bought for the specific bike/model, had a completely different conical chamfer at the end that screws into the caliper, meaning it would never seal properly. You’d bleed the brakes, get them all solid and a little while later, or next day, they’d need bleeding again. It clearly took a while for air to get past the thread and the ill-sealing chamfer. Once the replacement bleeders had been thrown away and the correct ones fitted, there were no further issues. Given the age of a 748 now, I’d imagine there’s a good chance that the bleed nipples have been renewed at some point, but had managed to seal somehow, or perhaps after much effort, until disturbed.
To prevent that possibly happening I've got into the habit of wrapping a little PTFE tape around the bleed nipple before threading him on.
I also wrapped gas type PTFE which is thicker around nipple to seal and aid the vacuum sucking process to reduce air bubbles but to no avail in my case. Did help with the vacuum bleeding process though.
