spongy brake pedal

[retamt]

This is 2000 century with 3.1 V6 engine. After getting major brake work
done at local shop the pedel is soft. It has had following work done:
rear cylinders,front pads,rotors,master cylinder, rear adjusted. Two
different shops have bled brakes. Fluid was flushed.
What else could I do? Im at my wits end. When applied and hold pressure
to pedel it will keep sinking to low position. Any ideas anyone???

[Blue Bowtie]

Internal master cylinder leakage would cause that. If there are no apparent external leaks, and the hoses aren't swelling under pressure, that's about all there is left. Does the car have an antilock brake system?