I have gotten more than a few Mc's that were bad out of the box.
however I have all new brake hardware on my jeep and SS lines.
I have a squishy pedal too
I had a similar issue on an older subaru that I solved by installing a MC with a larger bore. 7/8 to 1" you had to press the pedal harder but it was rock solid and would lock all 4 like nothing.
Not sure if this applies to you but there are 3 different bore size wheel cyls for the rear of a 90.(according to oriellys) my thinking is there is/was different options for the MC to compensate for the different bores sizes. but over the years they just started making the one option
however I have all new brake hardware on my jeep and SS lines.
I have a squishy pedal too
I had a similar issue on an older subaru that I solved by installing a MC with a larger bore. 7/8 to 1" you had to press the pedal harder but it was rock solid and would lock all 4 like nothing.
Not sure if this applies to you but there are 3 different bore size wheel cyls for the rear of a 90.(according to oriellys) my thinking is there is/was different options for the MC to compensate for the different bores sizes. but over the years they just started making the one option