I have a feeling the caliper slides are the culprit. I would rec to replace the front calipers. But first, you are going to have to pin point what is causing the spongy pedal. This is done by clamping off all four brake lines at the hoses by each wheel. Start the vehicle up and check the pedal. Chances are that it is nice and firm. If it is not, the master cylinder is at fault and will need to be replaced (or there is simply air in the system). If the brake pedal is nice and firm, disconnect the clamps one by one and see what makes the most difference when doing this. If its in the front, than the calipers will need to be replaced (sometimes one of the caliper slides stick and the other one is free witch causes the caliper to flex when stepping on the pedal and results in a spongy pedal.) If its in the rear than something may be broke or they are way out of adjustment!
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I have been helping another gentleman with a problem very similar to yours. Even though his vehicle is a dodge, the principle is still the same. Please read through it and you will see that this procedure is working for this mechanic.
Well, it is up to you... but I would be happy to help you out if you have any further questions.
Let me know!