Well guys, thank you all again for your answers. The car is a 69 Hemi roadrunner with power brakes and front discs. All the components are rebuilt originals with the exception of new discs, pads, and braking lines, with no aftermarket parts. I love the idea of checking engine vacuum ( the engine has a replacement stock-type cam), since the plastic port that connects to the intake manifold has an extra nipple just for that, all I need is to buy a small gauge and see. Meanwhile I will remove the MC and send it for rebuilt, just to be sure, because most of you seem to agree with me that is the problem.