Hi,
I just replaced the rear calipers in my 96 A4 (they seemed to be sticking). After finishing up and bleeding the system, in my excitement over finishing, I forgot to put the cap back on the brake resevoir. I took it for a short drive to make sure the brakes were working and the pedal felt odd and somewhat spongey. I opened the hood up and was going to try to bleed the system again and noticed the cap was off. I replaced it and tried the brakes again. This time they felt solid and the sponginess was gone. The question I have what purpose the cap has (other than keeping the fluid in) since I thought it was vented anyways. This is just an FYI type of question.
Thanks,
Old Audi Owner

 

im surprised u had brakes at all... the cap maintains the pressure in the system...

 

As far as I know the cap doesn't keep pressure in the system.

It is there to keep fluid in and dirt out.

 

Me too.. Hmmm

it does need back pressure so thats why. but there usually a one- way valve in the line from the cap to the brake lines

 

welcome to the forums, alcadena! there is a one-way valve of sorts that keeps your system pressurized with the cap off. like david said, it's basically just to keep dirt out and fluid in..check for floating pieces of dirt just to be sure, but you should be fine.