Audi 80 avant TDI braking problems

i've got a 95 80 tdi which i've been told has the vacuum assisted servo.

the problem i have is that intermittantly the brake pedal gets very very stiff and the brakes are virtually non existant. the brakes also pull to the right when this happens.

I've checked all the lines to the servo / master cylinder and all seems ok with no obvious faults.

the problem then just dissappeared until today when it came back.

is this likely to be servo? or any other suggestions abs fault maybe? I'd like to be able to test a component rather than just replacing them all until i fix it!


cheers john.