[AT] OT--Farm Truck brakes

[John Hall]

 I have a problem with the brakes on my F-600 Ford grain truck. I have had 
the truck for a about a year but have never put it on the road. We have been 
making some odd and end repairs to get it roadworthy. The guy I bought it 
from had tags on it through 2007. He put a master cylinder on it some time 
back. I noticed an empty can of brake fluid in the floorboard when I bought 
it. While using it around the farm last summer we needed to add some brake 
fluid. Then this winter while sitting under the shed untouched for a few 
months, all the fluid disappeared from the master cylinder. We refilled it 
but had no brakes. Parked it to work on another day. When we went to get it 
to take to the shop, we had full pedal/brakes and the master cylinder was 
still full. I have looked on every wheel and cannot find a leak (didn't pull 
the wheels--with this much fluid missing it should be easy to find). We also 
pulled the vacuum line of the booster to see if it was leaking into it and 
burring the fluid--dry. Don't see anything leaking from the master cylinder 
either.  What is the chance that when the guy changed the master cylinder he 
got a huge air block in the line and was able to drive the truck for a 
couple years and not know it--and then it finally worked its way out sitting 
under the shed? If that is not a possibility, then where could all this 
fluid be going?

John Hall