Testarossa Brakes Require High Effort

I am new to this forum. I have a late 1986 Testarossa that is experiencing a brake issue that I believe is related to the vacuum servo, but I am looking for validation or any advice out there. Recently, the brakes need a higher amount of foot pressure to make them perform. The car has low miles on it. The rotors are in beautiful shape and I believe the pads are fine with no glazing. I am about to change the brake fluid, but would like to know if anyone out there has experienced a similar problem and whether it was related to the vacuum servo or something else that I am not thinking about. If it is likely to be the servo, what is a good source, or can it be serviced? Thanks in advance for your reply.