1995 Geo Prizm Brake peddle go down when stopping.

Make sure the rear wheel cylinders and the front calipers are not leaking, as well as the flexible brake hoses, and the steel lines. If they are OK then the master cylinder could have an internal leak and would need to be replaced.

If there is any air in the system, it will cause a low pedal. You might try bleeding the system, but the MC sounds like it is bad. Now would be a good time to flush the brake system.