Brake Pedal Hiss?

[MLimpert]

I was sitting in traffic stopped, and I pressed extra hard on the brake pedal while reaching behind the seat. I heard a steady hissing sound coming from underdash while I had this heavy pedal pressure applied. Is this normal? The brake booster seems to make no noise under normal driving and braking conditions, but I can repeat this steady hiss if excess pressure is applied to the brake pedal. Thoughts?



I've never had a vehicle with a hydroboost brake system before, so I'm not sure what to expect...

 

[Patriot_RAM]

Mine does it also. I've often wondered what it is, but I figured it was probably normal.

 

[nickleinonen]

hydraulic pressure going over relief valve pressure in the boost system? i don't know if there is a relief valve in the system though



all the vehicles we've had with hydraboost have made a sound like that when pressing hard on the. it's normal i think...