"How can I tell for sure it is the water pump"
Tighten up all the hose clamps and elimanate that possabilaty than bite the bullet and get a new water pump. Be careful taking those bolts out. Do a search, lots of water pump info in the archives and its an easy job unless you have a York AC. 
Get back if you have any problem, I just did mine last week.
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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug
"Lifes too short to drive a boring car"
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