Operating system: Windows-10-10.0.17763 64 Bits
Graphics card: GeForce GTX 780 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 418.91
Broken: version: 2.80 (sub 57), branch: master, commit date: 2019-04-16 06:07, hash: rBedc1b0167518
Short description of error
Weight Painting uses the Pose Mode keymap. Even though it's possible to assign keys to Weight Painting actions, they will not resolve if they same mapping is already used in Pose Mode. This makes effective use of the Weight Painting hotkeys impossible and does not follow the keymap hierarchy (Weight Painting is listed as a standalone 3D View category).
Exact steps for others to reproduce the error
Add a new event to the Pose (Global) keymap, listening to Keypboard Press F (e.g. 'screen.keyframe_jump').
Test Weight Painting mode for a skinned object with its armature selected. Radial brush size control has been deactivated in favor of the new Pose Mode event.