could be master, could be abs, could be calipers or hoses

 

If I understand you correctly, it seems to happen when the brakes are hot and doesn’t happen when the brakes are cool.

Just curious, what brand and type of brakes are you using?

 

if there sticking its usually a caliper issue or hydraulic

 

Update: Still having the problem but here is how it works: The brakes have a fair amount of travel when started cold. As I drive around and brake, the brakes (pedal) will get more sensitive and have less travel to stop, getting to the point where you barely need to touch it to stop. If I get on the highway quickly, the brakes will be fine but I usually don't need to touch the brakes much if I'm carefull. If I'm at higher speeds (60 and up) and the brakes are acting up, the steering wheel vibrate pretty badly. If I stop the car when the brakes are acting up (sticking?) and turn it off and pump the brakes a few times (to release the vacuum) and restart the car, the brakes return to normal brake travel. With use, it will get more and more sensitive again. The booster seems to pass all the tests I have seen online for proper function. The check valve seems fine as well.

I was planning to replace the master cylinder, but am wondering if I should replace the booster as well. Would prefer to just do the master cylinder and not have to dig in further but don't want to end up doing it twice if only the master cylinder doesn't do the trick. Could a spring be broken and a rod is not returning in the cylinder or booster?

Any thoughts? Has anyone had similar issues? Thanks in advance for the help.