Brakes drag when hot under the hood

Whenever it’s hot under the hood my brake pedal gets hard. It’s weird because as long as I’m on the highway or it’s cool outside it’s fine. The hotter it gets the brakes will start to drag. It’s real bad when I get stuck in traffic. If I drive down the road for a while everything goes back to normal. Same thing if I shut the car off for a while and let it cool. I tried flushing the brake fluid and nothing changed. I’m guessing it has something to do with the brake booster or master cylinder but wanted to see if anyone else has ran in to this

 

Could be a crack in the vacuum boost line. Moving at speed might create enough vacum to overcome the leak but not at idle. Could be the booster itself

 

It’s not a crack in the line. I couldn’t have got that lucky. I think I’m just going to replace the booster and master cylinder then put on stainless brake lines.

 

can you check what is you vacuum level when it get stiff