I just purchased my 92 vette a couple of months ago I don't drive the car much (maybe a couple of times a week)

After the car sits for a couple of days the brakes are very soft for the first couple of miles then is fine. Normally I would assume the master cylinder would be the cause but it is only for the first couple of miles of driving and if I drive it everyday I don't have the problem. I am worried it could be in the ABS or the ASR system and it may get expensive quick, can anyone give me alittle direction here. The ABS and the ASR work fine all of the time.

losing vaccum when it sits?

No I don't think it is a vacume concern, The pedel goes real low like the brakes need to be bled but after a few miles they are fine again. This only happens after the car sits for a week, if I drive it everyday it is fine.

hydraulic leak for sure