I took Stan Pittman advice and topped off the system with a can of refreigerant (13 oz) and all is well now. I know I must a slow leak somewhere but it probably will be too dificult to find out. It leaked over the winter. When it gets worse I will try to find out where it is leaking.

I did not check any pressures I only found the low pressure side or the gas side of the system and added the reregerant after bleeding the lines a bit to minimze air/moisture injection into the system.

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2010 ML350 Bluetec
2012 Mustang Convertible