Derek,
The drop in idle is most likely caused by the friction of the throwout
bearing pushing up against the pressure plate fingers. Could be you have a
heavy duty pressure plate with high spring rate - good news; OR your throwout
bearing is seized and will soon disintegrate - bad news. Enough joking; I
honestly don't know if the throwout bearing is of the roller bearing type or the
plain bearing variety,; the later is just a round lump of machined steel- no
ball or needle bearings anywhere in sight. I just replaced the clutch in my
MGB-GT and was surprised to see this kind of throw out bearing. It too drops
revs when I'm on the clutch.
The spongy feel is either a hydraulic leak in the system, or air in the
lines. Bleed it while continually topping off the clutch master cylinder. You
might also have a helper pump the pedal while you look for seepage from the
lines and the master and slave cylinders. DOT 3 & 4 brake fluid absorbs water
vapor from the air. After a while, corrosion forms in the bores of the
master/slave cylinders which causes leakage past the seals. A solution which
has been suggested to me is: Hone the master/slave cylinders, replace the guts,
flush the lines with alcohol, and then fill the system with DOT 5 fluid. DOT 5
fluid is silione based and does not absorb water. It is however expensive and
it is a bit more difficult to bleed the air out of it. Once its done though, it
is a permanent solution.
Hope this helps,
Brian Corrigan
67 Series V project car
72 MGB-GT daily driver
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