I use metal tubing (brake line) for supplying vacuum to the modulator,with short hunks of fuel hose to connect it to the manifold fitting and modulator..using only cheap vacuum hose alone often leads to this issue when it either collapses under vacuum after it gets old,or splits where it goes on the fittings --fuel hose withstands the heat and wont collapse or split as easily,the brake tubing prevents any chance of a hose collapsing under suction.