what you need is a slack tube manometer. test off blow by tube, you can make one for less then $10 or buy one homemade versions can be found in a web search, either work fine
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7506600348

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74 240D rolling restoration, daily driver
ASE Master Truck Technician
I enjoy my work so much I had to buy a diesel to play with !