Hard break pedal

It’s most likely the plunger in the foot valve is seized up. You can try pb blaster or similar to free it up. I wouldn’t trust it unless it breaks loose easily. Depending on how it’s mounted they can be a pain to replace.
@Heavyd @spsauerland @pushbroom may have some other ideas.

 

Those older brake pedals are fairly common. Should not be too difficult to free up or replace at shop.

Its really important to have it working right. Lives depend on it without saying.

 

Spray with a good penetrant. Work it in by trying to move it up and down over and over and hopefully it frees up.

 

Myself I'd just change the treadle valve. If the rubber seals are hard enough to cause the plunger to stick its probably on its last legs anyways.

To me, brakes are one of the most important things on a vehicle. I wouldn't skimp on them.