Hi peeps

I have posted up here before about the brakes being really spongy. Now they are spongy again and I don't know what has done it this time

The first time the car failed mot with naff brakes the rear shoes were changed and the cylinders were replaced aswell. The brake system was also bled and the brakes seemed ok.
A month or so later the car failed mot again with shit brakes. So the master cylinder changed and the brakes were bled again.
Car eventually got through mot but the day after the brakes were spongy again!

Now I haven't got a clue as to why they have knackered up again and I don't have a clue as to how!
There seems to be no leaking as the reservoir is at the same level all the time. I have been told that the ABS could have something to do with it! How?
If I had lots of hair, I would be pulling it out by now. But as there isn't much left I have to suffer lol

The car is a MK4 Cabriolet with an RS conversion done before I got it.

Any help/ideas/hints/tips etc would be greatly appreciated, even if it means how much petrol to use! lol

Thanks peeps