only 2 places it can pull air in at
-master cylinder
-wheel cylinders
normally...
unless there is a crack in a line or a fitting leaking somewhere

being an XL2 model it has some age on it
my guess is to pull the master cylinder boot back and see if there is any fluid inside the boot, also pull the wheels and check the wheel cylinders.

AND... even if you do this and see no fluid present it still can pull air in if the MS plunger seals or wheel cylinder seals are dried out from age.
when they get old they may shrink up or just be worn enough to allow air to get sucked in when the pedal is released, therefore is where the air is coming from.

Checking the lines for leaks is a bit tricky though, you'll need a pressure bleeder to do this to apply pressure, if there are any cracks or loose or damaged fittings the pressure will make it show up.
If you find no issues during that then the problem has to be in the seals.