Brakes

Probably the master cylinder. The gasket around the piston goes out and when you step on the pedal, the brake fluid seeps back, through the piston gasket, behind the piston. On a thirty year old vehicle, if you intend to keep it, replace everything. Master cylinder wheel cylinders etc. and bleed accordingly. Parts are not that expensive considering the ramifications if they go out while driving.

Keep in mind brake fluid can attract water. The piston in a master cylinder only travels a short distance within its bore to stop a vehicle. The back end of the bore can become rusted because the piston never reaches that far, so corrosion builds up. Pushing the pedal all the way to the floor can destroy the gasket around the piston, causing it to leak and allow fluid to pass back behind the piston, reducing pressure to the wheel cylinders.