Squishy Front Brake Lever

I have had a weird experience.
Whilst replacing my tyres I took the opportunity to strip down the brakes for a good clean.
I cleaned them out no hassle. Bike went back to its usual wooden self.
Last week I decided to upgrade the pads as I noticed one pad out of 4 was a lot more heavily worn.
This time I worked on all pistons and discovered 2 must have been sticking ever so slightly. Sorted that.
Upon completion took the bike for a spin, spongy as anything.
I strapped up the brake lever for 24 hrs.
There was a decent amount of improvement, still not enough.
I then bled the master cylinder only. Hey presto the play is just perfect far better than I have ever had it.
I fitted SBS pads which are owned by Brembo. For under £30 a side.
Here is a pic of the pads. Now they are bedded in they feel much better than the original Brembo pads. I have attached an image of the pads.

 

Just recently fitted the exact same brake pads to my Speed Triple.

 

The zip-tie trick works well on my Ninja 300, but only worked for <24 has on my wife's GT900 front brake. Can't find how air is getting into the system,,,yet,