Remove the regulator handle clip using a rag as shown.

Inner handle removal/installation
Regulator handle installationMazda 6 Service Manual: Abs (abs/tcs) hu/cm removal/installation
Caution
When replacing the ABS (ABS/TCS) HU/CM, configuration procedure
must be done before removing the ABS (ABS/TCS) HU/CM. If configuration is
not completed before removing the ABS (ABS/TCS) HU/CM, TCS will not work
properly (with TCS model).
Do not drop the ABS (ABS/TCS) HU/CM. Re ...
Mazda 6 Service Manual: Dual proportioning valve inspection (without abs)
1. Connect the SSTs to the brake pipes as shown in the figure.
2. Bleed the air from the brake system.
3. Measure the fluid pressure of the master cylinder and the rear brake.
If not within the specification, replace the dual proportioning valve.
Fluid pressure
...