Welcome to JA!
If there is no fluid leaking then the Brake Master Cylinder is failing.
This is a cheap fix and will have you stopping like new :) Here is a link to a replacement Master Cylinder.
Removal:Drain the Brake Fluid.Remove the Brake lines from the CylinderUnbolt the Cylinder from the Brake BoosterSlide Cylinder out and remove from car.Installation:Slide New Cylinder into Brake BoosterInstall the Cylinder BoltsConnect the Brake linesFill with fluid and bleed the brakes :)
Please let me know if I can assist you further :)
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