I had a broken connection on my vent solenoid, replacing it fixed part of the problem but the CEL would still come back.
After using SEAFOAM in the gas tank and brake vacuum line, the CEL went away for 1,000 miles so far
yes, you should be able to blow air thru when open, no air when closed. The problem with the old valves is that "gunk" can get inside and coat the inside of the valve and it won't "fully" close. So after I changed my valve, I had a smaller leak from the other old valves in the emission system. I could tell it was getting better because the time between CEL was longer. So I tried cleaning my emission lines with Berryman B-12 (no difference) and then SEAFOAM. After the first bottle of SEAFOAM in the gas tank, the CEL didn't come back for 200 miles. When it came back, another bottle stretched the CEL to 600 miles. Third bottle and I am up to 1000+ miles. So far the SEAFOAM is working for me.