Awwww, Deb. I read Gazza,s post and said "ah-ha", perhaps thats it. Then I read yours and back to the same problem.
How old is yours Deb. Perhaps the newer or older models are as Gazza describes. Also you mention yours is a 3MD. Is that the same model as mine of the 3MDQX-SC, which is a higher flow, lower head model. It would have 1in. in and out. Mine is 3/4 in and 1/2 out.
Doug, they are great pumps. Run for ever. Parts are easy to buy. I know they can even be repaired in Winnipeg. I only like the fan cooled models. I am also a huge fan of the Mak 4 pumps. Excellent pumps and fan cooled. Quieter than my 3 MD.