Sounds more like an internal leak in the master cylinder to me. Rebuild or replace. May need sleeving for a rebuild depending on bore condition.
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i