In most racing titles, the analog axis must be calibrated. On some titles and platforms the settings are remembered, and for others it must be done every time the sim is started. In the same way the wheel turns to full left and then full right so the game and the wheel knows the full extremes, you manually have to do this on the pedals. With potentiometer based pedals this happens within the first few seconds of racing so most people don’t give it much thought, but a load cell brake requires heavy force to reach its maximum so doesn’t get done “accidentally.” If it doesn’t get pushed to its maximum, then 100% braking is considered to be whatever the highest amount of force the axis has seen since it was started. Next time you get into the sim try pressing the gas just a little and you will see it goes to 100% also. You don’t need to go to the calibration screen to do, you just need to press all 3 pedals all the way. You might want to use the calibration screen the first few times until you get the “feel” for what 100% brake is.
To find 100% on the load cell brake, first open the calibration screen and squeeze down on the pedal and then back off just a tiny bit. If the indicator on screen comes down when you back of the pedal then you have to squeeze harder. Repeat until backing off just a little does not lower the indicator. Continue to back off the pedal and 100% braking will be when the indicator just starts to go down. Essentially, if the pedal is capable of 70 lbs maximum then you want to press down with 75 lbs, and then back off to 70 lbs. The indicator should read 100% at 75 lbs and also 100% at 70 lbs.
Don’t be concerned about pressing 70 lbs at every turn since you usually don’t need to apply 100% braking at every turn.