I've got the same problem and I'm sorry to see that no one has responded to your post. This might be like the "blind leading the blind" but I'll tell you my thoughts. Hopefully, some real expertice will chime in. I have read elsewhere that there might be debris in one of the valve seats and you can just clean them out. On the left side (top and bottom) you can see the square ends to couple of plugs. Just unscrew that and take a look at the valve seat and make sure nothing is in there like dirt/s...
