There is a thread on Van’s Air Force about carbon build up in the breather tube.  I pulled the tube off and there was some build up (maybe 10% of the cross-sectional area of the tube).  Most could be easily removed with a pick, but I used a wire brush to get the inside of the tube totally clean.  Here are all of the carbon bits that came out of the tube.

If this tube were to totally close off due to carbon buildup, it could pressurize the crankcase and blow out the front oil seal.  To prevent this, I put a T in the breather line and ran that to a second vacuum valve positioned above the exhaust pipe to burn off any oil that might drip out.