Why don't rules applied inside ContourPlot3D work?

The output of the following code is the same as the input.

waals = (P + a/V^2) (V - b) == R T;

ContourPlot3D[waals /. {a -> 1, b -> 1, R -> 1},
{P, 0, 100}, {V, 0, 100}, {T, 0, 100}]


But

ContourPlot3D[(P + 1/V^2) (-1 + V) == T,
{P, 0, 100}, {V, 0, 100}, {T, 0, 100}]


works fine.

What's wrong? Do I have to specify all the parameters in the equation?

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– Michael E2 Aug 9 '13 at 4:02

ContourPlot3D[

As suggested by @rcollyer, ContourPlot3d has HoldAll attribute, which can be seen as ,Attributes[ContourPlot3D] (*{HoldAll, Protected, ReadProtected}*)
The more complete answer is it has the HoldAll attribute. – rcollyer Aug 8 '13 at 19:22