According to the Rock Auto Parts catalog, the Seville normally
has 2-15/16" pistons for the fronts while the tried and true '80s GM Monte Carlo size car has a 2-1/2" piston. The Sevilles's are then 7/16" larger in diameter. I know you said that you have the e-brake calipers, which are supposed to be 2-1/2 - maybe - as it does look like there are some with the 2-15/16 rears just to confuse the issue
you have the 2-15/15 version, this calculates as each front piston having 4.9 sq in of surface area with the rears at 6.77 or about 25% (round numbers) more surface area. A physics law(Boyle's) - "Pressure and Volume are indirectly propotional to each others. if you increase the Area the pressure will be decresed, and if you decrease the area of the applied pressure, the pressure will be automatically increased. Hence, pressure is indirectly propotional to area."
This tells me you might
have a problem in that the rear caliper pistons may
be too large and not giving you enough brake power. In actuality, those calipers should probably be in the 2 to 2-1/4 diameter range
Can you tell - I'm being cautious. Too dam' many variables when you deal with OEM 'stuff' adapted to our hobby cars (I really like my aftermarket Wilwoods