I changed my front calipers last weekend, bleed the lines, and noticed a hissing sound coming from the master cylinder area shortly thereafter. I finally had the time to look at it more closely today since I didn't saw any fluid leak or loss of braking ability at the time. After feeling around the area to find where the air leak was coming from, I found that the hissing sound was coming from in between the master cylinder and brake booster. The hissin sound would go away a few seconds after turning off the engine. What can be the cause of this problem?

 

No one ever had this problem before? I'm thinking there may be a seal between the master cylinder and booster...

 

your assumption might prove to be correct.

there are lots of seals in the brake booster.
if one goes bad, you could be searching for a leak/air in the lines for days

 

I also had a problem with my brake booster, I got a used one from car-part.com.

https://acurazine.com/forums/1g-tl-problems-fixes-118/brake-booster-help-786282/

 

Mines still work normally, just loose the ability to brake once car engine is off.

 

When I had this same problem the car will still brake but the pedal will go all the way down to the floor and also won't stop like right away.