Pretty common problem unfortunately, the seal on the brake booster is probably compromised, and the brake vacuum pump has worn out. Regardless, the vacuum pump is probably not coming on, perhaps because it isworn out. I'll find you a link. Here is a thread with full discussion of possibilities from ...

That is a little strange it works once is warmed up. How cold is the weather there? I assume the car was recently purchased? My first thoughts went with a frozen brake fluid if weather is sub zero, from either water in the fluid or wrong fluid altogether. I've had this experience on two other cars, ...

That is a little strange it works once is warmed up. ... I think not unusual, what OP describes was exactly the symptoms when the pump was bad on our S60. Probably not very unusual it actually makes sense. See jimmy57's post for much more insight but basically once the engine warms up and develops ...

If you turn ignition on but do not start engine you should be able to push and release brake pedal 2-4 times and then you should hear the vacuum pump run for 3-10 seconds and then go off. Pushing brake twice should make the pump run. The switch is behind left headlight with two wires in a brown conn...