Brake hoses failures of this kind are actually pretty common. What happens is they either swell or tear inside. The pressure of the fluid when the brakes are applied is more then enough to press by this, but the return pressure from the brakes is not enough to and the brakes stay applied. These type failures usually show themselves after some driving. When it does happen, quickly determine which wheel is the problem and loosen the bleeder for that wheel. If the fluid comes out under any pressure and the brake realeases on that wheel, then it's a good bet that's your problem.

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85 300CD
83 300TD
78 240D (daughter)