do you guys know of any reason why my brakes are squishy only when it is hot outside? it did it last summer lightly but when it was cold for the winter they were fine, anytime above 15 degrees when i drive for a bit the pedal goes down when breaking. it doesn’t go down when the car is off though? i can’t find a vacuum leak if that would be affecting it.
I have the same problem with my TEG. I was told that its an issue with the master cylinder, that it expands in hot weather due to age.
Does the pedal sink if you’re resting on it when you’re stopped? That would indicate a leak somewhere.
If not, it could be air in the lines. DOT3 and DOT4 brake fluid (which most of us use) is hydroscopic, meaning it absorbs moisture. This lowers the boiling point of the fluid which can lead to vapor forming in the brake lines. It’s a cruel snowball effect, too. The more water it absorbs, the easier it continues to absorb more water. They say you should change the brake fluid every year or two, but who the hell does that? However, if it’s never been done or you don’t know when it was done, try that before you start really tearing stuff apart.
get a new mc and flush the brakes out with valvoline syn power 
probably the old brake fluid having moisture in it. getting boiled to fast if it only when its hot. try changing it out. most likely if you have done it the master cylinder is going to go out afterwards from all “dirt” in there. some shops should have a brake fluid flush machine the would make it easier.