Hello!

A buddy of mine told me to use two of these valves => one for the front and one for the rear.

Now I have a bad braking function. The brake pedal travel is half the way to the floor and the front brakes will not look up at a speed of approx 25 mph. I think the is no air any more in the system.

But I can`t understand why there is a proportioning valve for the front brakes installed at the original setup. Or what is the function of the part installed at the master cylinder in the line for the front brake? (Sorry I tried to post pics but without success)

what brake setup are you running?

Did you read this? http://www.camaros.net/forums/showthread.php?t=89525

The factory valve in the front line is a "hold back" or "metering" valve. It will not allow fluid to pass until aprox 50 psi is reached. This allows the rear shoes to start contacting the drums overcoming the springs. It only helps with very light application of the brakes, like when you are on snow or ice.
David