The brake booster and the brake master cylinder from my skoda favorit was disassembled previously and i put it back together.. when i completely depress the brake pedal i hear a hissing sound as if air is escaping. when i am in the engine bay and someone presses the brake i feel the air escaping. The brake pedal goes all the way to the floor when the engine is off even though it has some pressure. What could be causing this? When i was reassembling them i only saw one seal that goes between the master cylinder and the booster. It is a flat O-ring with a small portion cut out. something like this:

The slit was turned up to a marking/indentation on the brake booster.

Is there supposed to be another seal? Was the seal put in correctly?

There is no brake fluid inside the brake booster... no leaks in the vacuum hose... so i suspect it must be the something to do with the area where the master cylinder and the booster are joined..

Please offer some advice/ assistance

Flipper.

Firstly are you sure you have bled the air from the hydraulic system? Even if the servo is not working you should still have brake pressure

Firstly are you sure you have bled the air from the hydraulic system? Even if the servo is not working you should still have brake pressure

i'm pretty sure i did.. went through 2 bottles of brake fluid.. but what would explain the hissing sound and the escaping air?

The plastic thing you posted a pic of is just there to make sure that any brake fluid leaks from the rear/end seal of the master cylinder drip down the indentation on the front - instead of going inside the brake booster. So I believe it should be installed with the gap facing down, this is correct. The brake booster should have it's own seal around the output rod (probably not the correct technical term but you get what I mean), that it sits in front of.

The other thing is ISTR the Favorit has a quirk in the way you are meant to bleed the brakes. You're not meant to do it longest pipe to shortest in order as is usual with most cars (although I forget the correct order - can try look it up if you're struggling). I'm guessing this does not apply if you managed to get brake fluid out of all the bleed nipples, but ISTR, with the pipes disconnected from the master cylinder, having to get someone to press the brake pedal, plugging the holes with my fingers as they released the pedal, and repeating - to get the brake fluid to flow through at full pelt.

A hiss is normal during operation, and agree with Tom - if the servo had failed your pedal would be nice and firm. It may be that you feel air because there is no pressure in the brake system and it escapes in the absence of it.