Welcome to the site. Maybe you could share a pic of the car with us?

Have you even looked into the master cylinder? Like Glenn above, I'd be curious as to the color / condition of the fluid.

I am also curious as to the brake pedal length. I wonder if the builder change the mechanical advantage during fabrication by making the bends to place the pedal where he wanted it.

Could you measure the brake pedal? We's need to know the 2 distances from the pivot. Pivot bolt center to the connection to the master cylinder center and then pivot center to the pedal itself center.. The mechanical advantage is the "horsepower" you can input to the brakes hydraulic system.

My other thought would be to lift the car completely off the ground and have a helper apply full pressure to the brake pedal while you try and turn each wheel. I'd be curious if any of them turn. Even if you used a long prybar ( like a diggers rock bar) off the floor and really work at making a wheel turn.

Well, that's all I've got for now. We'll see what others come up with.