It's a 1991 940 with ABS, and fresh brakes hoses ~12 months ago.
I replaced the front/rear pads and rotors (due to warping), pushing the piston back with the bleed nipple loosened off. Pedal feel wasn't great after a test drive, so I bled the brakes using a vacuum bleeder.
I did all four corners, plus the nipple on the master cylinder.
A 20 minute drive with plenty of braking provided good pedal feel. On the way home the pedal suddenly felt way stiffer, and the car noticeably more sluggish, and it quickly got worse, so I pulled over and all 4 rims were very hot.
I used a spanner to loosen the master cylinder nipple again, into a piece of pvc hose, and it shot a decent amount of fluid, and the brake pedal went 100% soft.
Keep in mind, the fluid was only filled. Half way between min/Max. What have I done wrong?