Quick brake system sanity check?

[YakkoWarner]

I've just started noticing a faint sinking of the brake pedal when sitting at a light. My suspicion is the master cylinder seals are starting to go and allowing fluid to blow by. I do not see any reduction of fluid level in the master, and do not see any leaks at any of the wheels or along the frame rails. Being a full float rear axle means I do not have the ability to pull the drums for a full rear inspection, that requires pulling the axles out. I just want to run this by others who know more than me before spending $$$ on a new master cylinder if that is unlikely to be the issue.

 

[PrairieDrifter]

If you're not losing fluid, backing plates aren't wet. Then it's most likely the master.

Could try bleeding all four corners, then see how it feels after that.