Brakes not going off?

Hi.

I am having problems with my 1999 Ford Escort brakes.

The brake servo seems fine, but when I lightly touch the brakes they come on light, but do not go off. The only way to get them off is to stop and press hard on the brake pedal. BUT the other morning after a hard frost, when I tried this the brakes locked solid.

Any ideas?

I have read something in the manual about a non-return valve in the servo line. What does this do?