Try searching this forum for 'vaccuum' you can usually find a previous post on the topic you're looking for. Sounds like a leak or you need to install the check valve that needed to be added to most GC's.
Take a much harder look at your vaccuum hoses all over. I dealt with this during the heat this summer and found where a line going in through my firewall had been rubbing and finally developed a leak. The minute I had it patched the vents were working again. I patched it quick by using the crevice tool from WD/40 or PB Blaster inside to bridge the line but later a small piece of fuel line for a RC engine did a perfect patch over the break. Too dark now to get a pic maybe I can get one tomorrow.
And you're not gonna believe the check valve, I mean checking your oil is probably harder than putting in a new one.
http://www.twocanines.biz/images/vacuum_line_repair.jpg
http://www.twocanines.biz/images/check_valve.jpg
Tough to photograph, orient the valve so that vacuum is allowed into the engine block.