You should run a vented cap. Otherwise there is no path for the road tube to suck out the exhaust gasses. Like this one:

Oil Filler Cap (Breather Type) - Stant No. 10064


Deve

1950 Chevy 3100 Deluxe Cab
1950 Chevy 3100 Standard Cab
In the Stovebolt Gallery
The Think Tank
More info and tips at Deve's Technet