By "servo" I assume you mean the power brake booster.

Several things you can check easily:

Do your power brakes work OK?

Get any hissing noises from the booster, especially when you apply

the brake suddenly? (This is more noticeable on the 164 where

the booster is right against the firewall.)

Are the hose and valve from the intake manifold to the booster

in good shape? The hoses are bad about cracking, especially at

the ends and bends. Also there is a gasket where the fitting goes

into the booster that may leak.

If you have further questions, please identify which car and engine

you have as these make some differences in the system.