I don't know since when I have this problem, but break help is always on. Changing the setting doesn't make any difference. I can't make any of the wheels lock up, even when I apply full brakes.
The brake bar (when you look at the controls page) does go to the maximum setting.
I use a Logitech momo steering wheel. I tried reinstalling but that did not work. I did not always have this problem, and a few days ago for about an hour, it did work correctly.

Check if your brake pedal is correctly calibrated. Turn on the "show pedals" option (if you haven't already) and make sure during full braking only a red bar with a white line on top shows up in the lower right corner.

Send us your car setup pls.

I bar goes to maximum, no problem there. It happens with all 3 cars and all setups.

Send us a hotlap replay then pls..

I made a replay, if you want something specific, please ask.

I'm pretty sure you haven't set the brake strength high enough.

This was the standard setup, there is no difference at all between brake help on or off. And I thought this was different earlier, I think I couldn't brake maximum without locking the wheels. Could you test that?

I never got brake help to work at all. I just went into the wall.

I don't know, just... use a different setup with stronger brakes then
Many setups in LFS are set up in such a way that full braking doesn't lock up the brakes (even without brake help). The only thing brake help does (which isn't turned on for you, by the way) is that it releases the brake halfway when the wheels are about to lock up, but you'd be able to see that when looking at the pedal bars during braking.