No break-pedal pressure ONLY when outside temp is above 75 degrees and sunny

I have a problem about which the mechanics I have asked are baffled: In cool, cloudy or sunny weather, my breaks and break-pedal pressure are fine (the mechanics have said I have good fluid, brakes, etc.–they can find nothing wrong–and, of course, it never has a problem when they test-drive my car). However, when the weather warms up and it’s a bright, sunny day, my break pedal loses pressure and slowly sinks to the floor–a dangerous thing at intersections. There’s slight pressure if I pump the pedal constantly while at a light; but, without pumping, the pedal immediately sinks to the floor. This is a real problem, because this problem always occurs right when mechanics are not near or not open for business–and none I’ve talked to have ever heard of such a problem. The bright, hot sun’s affect seems to be key, because if it’s 80 degrees outside and cloudy, I don’t have the issue–but, then again, I don’t usually run the air conditioner if it’s cloudy. As such, I’m not positive; but, this may be tied to when I turn on the air conditioner–as though the two share the same vacuum system. Anyone have any ideas regarding this?

I would suspect the master brake cylinder. It may be marginal and when it gets hot the seals get too soft allowing fluid to leak. I would try another mechanic, this should be easy enough to check and fix. They could put a hair dryer or heat gun on the master cylinder to see if they can duplicate the problem.

Normally, when somebody describes a brake pedal sinking to the floor, it is a sure sign that the brake master cylinder is worn-out and needs to be replaced. But, if this is somehow tied-in with vacuum pressure, then it may point to the brake booster being the source of the problem.

However, even if the brake booster is the issue, you would still have brakes, with the problem being that they would require much more pressure on the pedal, and the pedal would barely yield when you press on it.

I think that this is almost surely a case of a worn-out master cylinder, but I think that you need to consult a new mechanic before you break something as a result of your brake problem.


How is the level of “BRAKE” fluid in the master cylinder ? if ok sounds like the master cylinder is flaky. Try this…very gently apply pressure to the brake pedal, does it slowly sink to the floor ? Now slam hard on the pedal a few times like in a panic stop. If this gives you some pedal the seals are worn in the MC. Do this with the engine running and in Park or neutral if a stick, and not going down the road !