[02v70t5]

Hi i hope some body can help, i have an 2002 v70 t5 with boost issues i fitted a gauge to see what was happening it idles at -18 and it pulls to 5psi unless in 3rd when it hits 10psi i've fitted a new boost control solenoid as it had come up on reading the errors which seemed to sort it for about an hour,
and last night i pulled the yellow pipe going to the wastegate and was getting a good 15psi all through but would drop straight off to 0. also since taking the pipe off i have a hiising coming from my brake peddle so a new servo seems in order could this have anything to do with my boostproblem?
thanks in advance