Ive had 3 disco 1s all had same prob all been servo take it off and examine it like i said mine hissed but not from under bonnet but where brake pedal connects

either shot master cylinder seals (issue before you changed it)
now air in system. clamping pipes as suggested may help, but depends on where air is.

is it an abs model? they can be tw@ts to get air out of pump.
is it a blue box master cylinder? new ones can be defective. check for fluid being pushed back into resovoir when the pedal is pressed.