A leaking diaphragm will often allow fuel into the crankcase and
the oil level will rise, smell like fuel and get really thin.
More likely it's a faulty check valve, or if the engine is one
that's flown for short flights, the spring in the pump has corroded and
broken, reducing the output pressure. The spring is on the crankcase
side of the diaphragm and the corrosive moisture and blowby gases can
attack it. If this is the case, the magnetos should be looked at as
well. Their impulse couplngs can corrode and their springs might break,
retarding the timing to around zero BTDC and lowering power to some
useless level. A real danger with the Bendix dual magneto used on some
Lycs. Both mags are driven by the one gear and coupling.

Dan