Brake problems and nobody finds anything

That’s a good morning for me. First of all, I’m sorry about my moose: Volvo V70 I, 2,4l 121kW or 165 hp, Bj 1999, Motor B5254S I have a problem. Sure, otherwise I wouldn’t write…. It’s about the brake from my moose. The problem only occurs once in a while. And even if my mechanic (no Volvo workshop) is NOT sitting in the car and I could demonstrate to him how it feels. So (for the cases in which it happens): When I drive a track (so 3-5 km) and over 50km/h and I step on the brake (seek, not fully step through to slow down the car), the brake pedal can be passed for about 1cm. Then I have to get off the brake. If I don’t feel like I’ve hit on a thorn, it’s not like the brake force or effect again.