A hissing sound as you press the pedal is not abnormal, but if it
continues to hiss as you hold the pedal down, you have a leak somewhere.
  If the van stops with a reasonable force on the pedal after you pump
it up, the booster is basically working.
  You didn't mention adjusting the back brakes.  Generally the need to
pump indicates the need for adjusting, unless there is air in the
system.  If the pedal feels spongy after pumping, more bleeding is
needed.  I seem to remember a post that mentioned the possibility of an
air bubble in the master cylinder that sometimes is hard to remove.

good luck, hope this helps
Larry A.

Michael Brown wrote:
>
> I just got an 85 Vanagon.  The brakes would take 3 pumps to pump up, so I replaced the master cylinder.  After I replaced it, I bleed the brakes.  Now when I push the brakes, I hear a hissing noise coming from the booster and still takes one pump to get a firm.  I pumped the brakes and held them down with the engine off, and when I turned the engine on the pedal depresses slightly and holds (bently test). When I turn the engine off, the hissing stops from the booster (holding the pedal down).  When I press the brake the rpms on the engine go up. I should also mention before I changed the cylinder and before I bleed the brakes, I did not notice the hissing sound.  The rotors, pads, drums, shoes, etc. are good and there is no break fluid leaking.  I would like to solve this tommorow.
>
> Does this mean the booster is bad?  Or is it a vacum leak somewhere?  Could I have hit a vacum line or something when I changed the MC?  If it is a vacum leak, where/how do I go about finding it?  I have an 81 westy, is the booster the same as on a 85?
>
> Thanks,
>
> Mike
>
> 85 Vanagon
> 81 Westy
> 72 Westy