As I understand it:
On early aircraft the alternate brakes are operated by hydraulic lines which actuate the selector valves. IFix Planes' diagrams show this very nicely.
Sometimes air can get into these lines and then pressing the brake pedals will not create the correct movement of the selector valve and thus pressure shown on the triple indicator will be lower on one side than the other or not reach the correct pressure on both sides.
This used to called by Airbus rather confusingly the 'spongy brake check' which I always felt was not very accurate and was trying to hark back to the car braking pedal 'feel' when there was air in the lines.
Normal brakes on old a/c and both Normal and Alt on new a/c are electronically sensed (again refer to those diagrams!) and therefore will always create the demanded pressure.