My guess is one of the springs in the clutch assembly broke. You'll still get some pressure, but not enough for heavy use. Looks like time to pull it apart!
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Ian
Mine: 72 w/ k301, 3 125's, 1 w/ hydro lift, 38,42, & 2 48 decks, 2 42 front blades, QA-36a & QA-42a thrower, tiller, lo-boy 154. Also, LOTS of parts.
family's:2 105's, 2 106's, 149, 2 lo-boy 154s, Farmall Cub.
Non-IH: 1940 JD Model A, 1954 JD 40 U, 1955 JD 40 Crawler, 2 1956 JD 420 U's, both w/ Henry Loader and Backhoe. JD 110. Wheel Horse (model unknown.) Power King 1614. We love our tractors! 
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