Cherokee 180 soft brake - causes?

I have a 1971 Piper Cherokee 180.

I changed the oil earlier this week, and then taxiied around the
airport as part of my runup check.

During this process, I noticed that upon first application - the right
brake is soft. Pumping the brake pedal gets the pressure back, and
then it works ok. However, if I taxied for a while, and then tried
the right brake again, it would be soft. Pumping the brake would get
it back.

The left brake is fine. The parking brake exhibits the same soft
behavior, but pumping the handle allowed the parking brake to work
too.

I checked the fluid fill level on the reservoir and it is full. I
have not had any brake work recently, so I am not sure how air would
have been introduced to the system. I also did not notice any obvious
leaks or drips of fluid at either the master cylinders or the gear
mains.

The plane does not have copilot toe-brakes, so that is one less issue
to fight.

Question: Are the symptoms I describe part of normal brake wear? Ie
the pad wears down, and now the piston has to extend further? I don't
understand why this would cause softness though...

I have wheelpants with the K2U fairings, so it appears it is time to
remove the fairings, dig deeper, and have the A&P look at it.