Series 3 brake problem

Hi I have a SWB 1979 with the servo on a dual system, red light on, on the middle of the dash and half pedal, rears bled and no air at all but can`t get anything out of front nipples, might be splitter but anyone with any ideas.....HELP!!!

it will be the pressure differential switch , located under the air filter, when you bleed on the dual systems it can force the plunger in the switch all one way (simulating a brake fluid leak ) you need to bleed the other brake line in the split system which is the front, by doing this gently you should be able to centralise the swtich and make the light go out

correction bleed the back first( as you cannot get fluid or air out of front the switch has cut this side of the system off to prevent further leaks) keep an eye on the warning light and stop when it goes out