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Ed, The problem is not trapped air; in fact, it's the lack of air that is the problem. In a closed system like the 2-seat Lancairs, the hydraulic fluid is trapped in a closed system. It has a low coefficient of expansion, but it does expand when heated. With no place to expand, the pressure spikes. The 4-series Lancairs have an accumulator (basically a bottle with a rubber membrane inside that separates compressed air on one side and hydraulic fluid on the other). This allows the heated fluid a place to expand...and compress the air in the accumulator just a bit; the pressure rise is probably not even noticeable. When you first service the system on a 2-seater, there is always a small amount of trapped air. Over time, this gets flushed out of the system, and the ability to absorb even a small expansion of the fluid diminishes...hence the pressure buildup. Hope this helps. Bob -----Original Message----- From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Ed Gray Sent: Tuesday, September 04, 2012 4:19 PM To: lml@lancaironline.net Subject: [LML] Re: Temp effect on hydraulics What is the coefficient of heat expansion on hydraulic fluid? Seems that big pressure changes would be caused by trapped air. Auto brakes don't fail in low or high temps. Could it be trapped air in the fluid line to the sensor? Ed Gray L360 Nearly complete with test hours, Dallas -----Original Message----- From: Lancair Mailing List [mailto:lml@lancaironline.net] Sent: Tuesday, September 04, 2012 5:01 AM Subject: lml Digest #3893 Lancair Mailing List Digest #3893 1) Re: Critical Service Bulletin by Silvio Novelli <ppxsn@novellisouza.com.br> 2) Barrett at fly-in by "Claudette Colwell" <colwell.ch@gmail.com> 3) Re: Critical Service Bulletin on Cruise RPM by Silvio Novelli <ppxsn@novellisouza.com.br> 4) Re: Critical Service Bulletin on Cruise RPM by Colyn Case <colyncase@earthlink.net> 5) Re: Thermal influence on Hydraulic Pressure by "Danny" <danny@n107sd.com> 6) Re: Thermal influence on Hydraulic Pressure by Frederick Moreno <frederickmoreno@bigpond.com> This digest is sent to you because you are subscribed to the mailing list <lml@lancaironline.net>. For archives and help click http://mail.lancaironline.net:81/Lists/lml/List.html LML website: http://www.lancaironline.net/maillist.html -- For archives and unsub http://mail.lancaironline.net:81/lists/lml/List.html