I posted this elsewhere a month or so ago, but its relevant here.
I had a repeat of an ABS issue on a 307 recently & I actually managed to scope it whilst it played up. The capture below shows a drop out of signal as I repeatedly touch the rear of the sensor......
Looking at the air gap, I estimated approximately 2.5mm clearance & after removing the sensor & scraping the corrosion away again the gap remained. I then decided to fit a small washer to pack the sensor out & reduce the air gap to approximately 1 mm.