Spongy brake pedal

[Davie b]

Hi guys ,
Can anyone help (S8 D2 1998) , I noticed yesterday that the brake pedal went really spongy , on pressing it i I felt the brakes bite then it went really soft with little or no brake bite to then firm up slightly at the bottom of the pedal swing .
I've checked the brake fluid and there is no sign of any leaks anywhere , the brake fluid hasn't gone down at all .
The only odd thing I can see is that the n/s rear brake pad has worn down completely , where the o/s still has about 3mm left on it .
Any ideas ???

Cheers
Dave