Assuming the system is properly bleed, you need to check rear brake set and adjustment. Too much rear shoe travel will cause this. Another possibility is a defective rear brake pressure regulator. When replacing the master cylinder one has to be careful that the brake light switches get filled with fluid. Some of them have enough capacity that if filled with air they will prevent getting a truly firm pedal.

Dennis  

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Mike Riley
Sent: Wednesday, September 23, 2009 1:59 PM
To: vanagon@GERRY.VANAGON.COM
Subject: strange brake problem

I have a problem with the brake pedal  travel, it seems to go about  3 inches then hit something mechanical and stop. problem does not happen with engine off, pedal stays high enough to not hit the obstruction. previous owner replaced master cylinder to no avail. anybody seen this before.
84 auto.
thanks
mike