Thank you for this information.
Yes, that's correct : I'm talking about this stock non-adjustable brake proportioning valve up by the radiator (on the left side, near the brake master cylinder). And as far as I know, the plumbing was stock before the mech modified it.
I, too, can't explain what could have caused the issue, as it doesn't seem to have a different action between left and right side.
The fact is, the issue has disappeared, and I wasn't able to reproduce it : maybe there was something wrong in the stock lines that have been deleted ? I really don't know...
Matt