'69 K10 - brake pedal sticks?

[JJorgensen52]

I typically drive my truck on the weekends, it lives outside yearround. Today I went to drive it and when I stepped on the brakes, the pedal felt super stiff and then stayed on the floor until I pulled it up with my foot. In the past it's felt stiff after sitting but after the first few uses, that goes away.

My truck was originally manual brakes, I converted it to power using a factory 3/4 linkage and booster from a JY truck (booster since replaced) - I still have 4 wheel drums.

Any idea what could be causing this? I tried lubricating all the linkage on the firewall without much change. Could the upper pedal pivot be corroded or sticking? My windshield does leak a bit when it rains.