[Snake2998]

Very interesting thread.

A rear brake problem should not have prevented the front from locking up unless it affected front pressure as well.

My guess at what was happening judging by the cure, is that the master cylinder piston for the rear brakes is too big for the 34 mm caliper so couldn't generate enough force on pad and hence locked up the whole master cylinder early in the stroke preventing enough movement for the front brakes to reach full pressure and work properly as well.

Going up in size of the rear calliper to 41 mm increased the rear piston area by 45% and hence pad pressure by 45% and movement of the master cylinder pistons by 45% allowing the front system to develop full pressure and and hence proper braking is the result.

All the vacuum booster is doing is making the pedal softer and pressure easier to reach but if you have hydraulic lock because of missmatched master cylinder/calliper piston sizes improved booster operation would not have helped much.