Do you hear any funny noises when applying the brakes (such as the ABS motor or valves operating). If not, then you may have air in the master cyllinder or lines. If it gets "spongy" after driving a while, at what speeds do you get concerned. If low speed is the problem, I would bet on the ABS kicking in (you should hear and feel it if that is the case).

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.