Brakes 1990 Seville

Has anyone overcome the problem with the Mk4 Teves braking system pulling to the left on emergency braking?

This is a problem which must be common, as it is listed on my CD from Alldata, and shows the hours to replace etc

GM state, just put a new system in, @ $460 for the part alone.
I contacted Teves and they say go back to GM, they cannot help.

As it is only Hydraulics, and from my researching with hydraulic people, I assume it is most likely dirty or blocked valving?

Any ideas of suggestions?

Thanks

Guy