My '65 C60 did the same thing with the clutch pedal. Adjust the clutch back to IIRC 3/4" free play in the pedal (I'm sure someone here will correct me if I'm wrong) the idea being that the pedal doesn't go 'over center' as @78Buckshot mentioned, before seeing resistance from the mechanism below. I did get a new one of those master cylinders at NAPA seven years ago, it wasn't even expensive. Like $78. Of course, now it lists for $139.

https://www.napaonline.com/en/p/NMCM34404

Holy moly-I found my receipt for that!! The NAPA part number is M34404 and it still shows as available on their website. While you're there, might want to pick up a spare clutch slave, too: 73466 also showing still available from NAPA on their website.

Hope this helps.

1965 C60 school bus | 1967 GMC 6500 school bus