Evaporative emissions equipment

[8lugburby]

I'm having trouble with my tank getting into a vacuum on long trips and I'm sort of confused as to what the evap system consists of on this old truck. I understand how a modern evap system works, this one is foreign to me. No purge, or vent valve, no fuel tank pressure senor. The vapor canister up next to the radiator has one 1/4" hose which I tested to flow freely back to the tank sender, one directly to ported vacuum on the throttle body and one 1/2 or 3/8" barb which was just vented to atmosphere with a simple baffle over it in my application.

I've used a hand operated vacuum pump to verify all lines and canister itself flow freely. does this truck use a vented gas cap? what exactly am I missing here. truck is a 1991 r2500 suburban, with California emissions equipment RPO code. from what I can tell the California emissions simply consists of plastering a pellet style cat directly after the muffler, kind of funny. no air pump or anything.

 

[gmbellew]

make sure the lines are hooked up correctly at the cannister and throttle body.

gas cap is supposed to have a check valve in it to relieve pressure or vacuum. so it could be a bad cap.

 

[8lugburby]

thanks for the info. exactly what I needed.

so a regular "emission control" style gas cap will have the proper relief or is it something particular to the older model?