The Emglos do just that, my cheap one just vents until it hits cut in pressure. Not buying one of those again. Sticking with the quality.Date: Wed, 28 Apr 2004 21:09:32 -0600 From: bearbvd@domain.elided (Greg Hermann) Subject: Re: [ihc] Air Brake Systems They generally do NOT vent compressed air. They 'unload' the compressor by holding the intake valves open full time. GReg