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The makers of the rubber pieces that are used in brake systems recommend that if DOT5(silicone) fluid is used then all parts should be new and the system should be flushed with solvent to remove all traces of glycol based brake fluid. The reasoning is that the DOT5 stuff is non-absorbent of moisture and the DOT 4, 3 etc. is. The mixing of the glycol and silicone in the system will allow moisture absorption of glycol to get moisture mixed with the silicone and this is not good. This leads to a high viscous sludge according to someone I know that did this to a classic car. It is almost impossible to get all traces of brake fluid out of a Hydraulic Modulator (the abs unit thing). There are pressure damper chambers and storage chambers that are not in the hydraulic circuit when you bleed. These areas have circulation when the ABS is active (a first roll test makes system valves and pump active) and would contaminate your best efforts at getting all the traces of conventional fluid out. The system on GM products of older vintage is simpler and may not have the storage chambers or maybe your dad is just lucky.

If you actually get the details on this stuff it is not really intended for active vehicles but is rather for race cars which will see high temps for extended periods and for classics which aren't used much and are big targets for moisture corrosion. Classics and race cars will have most of their systems installed as new parts.

Save your dough and periodically flush/bleed your brakes