My opinion? Air in the system. If all wheels bleed with no air bubbles there can often be air trapped between master cylinder and booster pump.
Try bleeding the line between these two to see if that helps.
94 XJ40 VDP
Oyster Metallic/Doeskin on Aero Alloys
220,000 Kilometers/136,700 MilesDaily Driver
94 XJ40 SOVEREIGN
Diamond Blue Metallic/Barley
111,000 Kilometers/69,000 MilesParts Car