I am sorry that you have not been helped yet. I just came online and saw your question.
You dont mention if the fluid was low or not. If it was, we need to check for a leak. If not then we need to do a brake hose clamp test. Use brake hose clamps to lightly clamp off each brake hose at the 4 wheels. Now test the pedal. It should be firm. If not, replace the master cylinder as it has an internal bypass.
If good, then starting at rear, remove one brake clamp at a time and retest pedal each time. At which ever wheel you remove the clamp and the pedal sinks, that is the problem area, faulty caliper or hose
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