Brakes

If you have rear drum brakes, a low brake pedal can be caused by the shoes failing to self-adjust over time. It is also not uncommon to find a lining that has become detached due to rust.

Since you are not losing brake fluid, there is likely no external leak, but there can be an internal leak in the master cylinder. You should see the red "brake" warning light turn on for that.

The clue to rear brakes out-of-adjustment is the brake pedal will get higher and harder if you pump it rapidly a few times, then it will be low again after leaving it released for a few seconds. The clue to the leaking master cylinder is it usually starts out as an intermittent problem. You will have a low pedal one time, then a normal pedal the next time. Pumping it rapidly does not change when the low pedal occurs.