Those are the Brakes

Hi and thanks for your responses in advance

My v reg 1.8 turbo octy has a brake problem

Solid pedal but sinks to the floor (under pressure) when engin is started. Noticed problem on my way home when the pedal almist hit the floor before the brakes operated. Doesnt seem to be air fluid lvl ok no sign of any leaks. I know what I think it is but would really appreciate some one who actually knows what they are talking about to give me their opinion. Also due to the age and low value of the car I dont want to spend large sums of money.

Awaiting your replies with anticipation

Regards Jason