You can modify it in the *.HDV-file of the mod.
Look for the mod-folder in the ../Gamedata/Vehicles-folder and search for the file with the extension "HDV".
Open it with a text-editor and find these lines:
HandbrakePressRange=
HandbrakePressSetting=
Normally they look like this for example:
HandbrakePressRange=(0.0, 0.10, 11) // intended for rally-type cars
HandbrakePressSetting=8
You simply can copy these lines so the default pressure will be 80% and you will have to set it up to 100% manually in game, or you change it into
HandbrakePressRange=(0.0, 0.10, 11) // intended for rally-type cars
HandbrakePressSetting=10
That should give the 100% as default.
But notice: a modified HDV-file will cause online-mismatches. So you better should make a backup of the original file.