Newbie Brake issues

[65 deluxe]

Hi Folks
I have recently added a 1980 928 s to my garage , Its been sat for a long time having covered less than 300 miles in 10 years and only 10k in the last 25 years . Its a euro spec car as I'm in UK and its RHD

I have had the usual starting issues due to fuel pump fuse and Ive managed to sort the electrics so that pretty much everything works apart from the drivers seat adjustment - It just blows fuses !!

Im struggling a bit with the brakes and could do with some help please from you knowledgeable folks .

I have taken all the clippers off - stripped and cleaned all surfaces and added copper slip to the places where they move along each other .
Pads and Discs all look good and Ive bleed the system well - However I seem to have brakes seizing on ( back 2 and right front ) and the brakes seem very poor , pedal almost soft and I have a sort of air hissing sound that comes from the front when I depress the pedal .
I have stripped it all down again and will replace the hoses at all 4 corners in case they have collapsed over the years of non use - I will also drain and replace the fluid . Im more used to older cars with no servo and wonder if I'm missing something .
any help would be gratefully received - Thanks Jon

[Mrmerlin]

from your info its time to make the brakes fresh,

replace all the flex lines with steel flex lines,
replace the MC and power booster and the valve that goes into the booster,

also replace the proportioning valves,
use ATE super GOLD fluid.

replace the calipers this should restore braking to full,

also consider swapping in larger rotors and calipers from an S4 on the front.

Dont cut any corners as it will mean you will be back at this issue within 1 year or less.