I was thinking similar as a next step. The issue would be the master cylinder piston not returning far enough to uncover the hole that allows the fluid to return to the reservoir. You could test this by opening a wheel cylinder bleed nipple and seeing if fluid will run through by gravity and atmospheric pressure in the reservoir. You can do better with a syringe and a bit of hose by pulling some through.
It's not unheard of for water to drain down from where the masters are fitted and cause the pedals to seize on their pivot shaft. Even if you pull the clevis pin out and the pedal swings up freely it could still be the master as the piston isn't physically attached to the pushrod.
More out of curiosity: It looks like a Canada spec car from your profile pic. Is it LHD with the master cylinders also on the left?