Brake Pedal Problem

[a.muqeet]

Hi, I am new to this forum and new Yaris/Vitz driver.

I just bought automatic Vitz 2001 last week and encountered a problem.

Its hot weather here and while i drive on A/C with the speed of 10 to 15 mph and apply breaks the pedal gets really hard.
While i turn of the A/C and apply breaks at same speed it works fine. I went to mechanic but he couldn't sort this problem out.

Any one here can give good but solid suggestions ?

Regards,
A.Muqeet

[PhillyG]

have someone check the vacuum pressure with the ac on and with the ac off... possible the ac being on can affect load and drop the pressure going to the booster. not likely.. but possible.