I have a 1982 Ford Granada. Is brake pedal fading always caused by a defective master cylinder? Or, can it be also caused by defective slave cylinders? Can bleeding and refilling the fluid cause temporary failure of the master cylinder (pedal down to the floor)immediately after refilling? After my mechanic replaced the front calipers he said I needed a new master cylinder as well.
If the master cylinder or wheel cylinders/calipers were replaced and not bled properly then you will have air in the lines. This will cause the pedal to travel to the floor.
Pump the brakes with the car in park and the engine running. If you can get a firm pedal after several pumps of the pedal then you have air in the lines.
Most shops do not use the old “pump the pedal” approach to bleeding brakes. But if your mechanic did use that method, it’s entirely possible to damage the seals in the master cylinder by pushing the pedal beyond its historical range of motion. That’s easy to do when bleeding brakes because no one wants to do a 100 short strokes so the inclination is to use nearly the full pedal travel during bleeding.
This is especially true if that is the original master cylinder on this car. Over ~28 years, a lot of crud has built up in that bore and that stuff can damage the piston seals if forced past that crud layer.
However, you should probably forget about assigning any blame for it now. Regardless of whether or not the seals were damaged by the bleeding operation, that master has served its purposeful life at that age and was probably better it failed now and was replaced. They don’t last forver and yours was likely well past its expected life.