  We show how the isospin vector and axialvector current spectral functions
rho_V and rho_A can be used to determine in leading chiral order the low energy
constants B_7^{3/2} and B_8^{3/2}. This is accomplished by matching the
Operator Product Expansion to the dispersive analysis of vacuum polarization
functions. The data for the evaluation of these dispersive integrals has been
recently enhanced by the ALEPH measurement of spectral functions in tau decay,
and we update our previous phenomenological determination. Our calculation
yields in the NDR renormalization scheme and at renormalization scale mu = 2
GeV the values B_7^{3/2} = 0.55 +- 0.07 +- 0.10 and B_8^{3/2} = 1.11 +- 0.16 +-
0.23 for the quark mass values m_s + m = 0.1 GeV.
