2004 S60 D5 Brake system problems (cold and wet weather)

[sane]

Two months ago I noticed a strange problem with brakes by driving - when I press the brake pedal, the car goes to the right. First I thought I was wrong, maybe causeof raod surface fault the wheels goes awry. Next weeks all was ok with brakes. But after then it happened again. And this is happening only then, when the car was staying overnight: on the morning, when I am driving that day first. On the control panel the computer shows nothing. My car master could not identify the problem, because the computer does not give any message. And he doesnt know, why is that strange feeling by braking. He suggest to change the brake fluid (I didnt do this till now). So some weeks all was good with the braking and I continued to drive. I was in dealer service too and they also doesnt know, what is that problem, because computer gives no message. In dealer service they said I should change the front brake pads - they become loose. I change it. Some time I drive ok and brakes was ok too. But one night was raining. On the next morning the problem was again by driving - braking. I called some friends, who are able in cars and they think it is ABS block: 1) electrical part; 2) the same ABS block. Today was +4 degree and the problem with braking was again, when I started my car. My car master also agreed with this, then drove in servive and could not find the problem with computer. Also visual inspection didnt give the result. After the service cheking my car master was driving by my car and this problem was not any more. I fixed that this strange case is happening all day (when at night was raining) by starting/turning off the engine, but today when is just +4 degree and no rain - it was only one time (after restart the car - brakes were ok). If someone can help me with an advices, where is the problem or someone had similar expierence, I would be very grateful. Thanks!
Sane