[chrismorse]

Progress is slow on restomoding my dad's 74. Recently, new PMB calipers, discs, (trued), bearings, SS hoses and new MC.
Mightyvac bleeding was slow, I suspected bubbles coming from a slightly loose tubing fit to the bleed screw - I used a tie wrap to snug up the tube to bleeder connection - still had a lot of bubbles, so tried teflon tape on the bleeder screw threads, (not on the taper seating area - don't want bits of teflon drifting about in the system). This combination made the mighty vac system bleed fairly quickly.

I have heard of potential air being sucked past the piston seals and wondered if that was a problem with old seals, or too much vacuum, so i do not really pull a lot of vacuum to do the bleed and this has worked well on many brake jobs.

While i was a service manager/service writer at a GM/Subaru/FIAT dealership inthe 70's, i was the designated pedal pusher for brake bleeding. During that 10 year stint, i noticed a significant, (not huge), number of master cylinder failures, - sinking pedal, inability to bleed, immediately on concluding the usual pedal UP/ down bleeding dance. My conclusion was that the MC seal was failing because of the full, long stroke of the MC piston going over rusty trash build-up in the bore, at the end of the normal brake MC piston travel, cutting the seal. I credit this trash build up to brake jobs being done very infrequently on the Detroit Iron and insufficient annual/bianual brake flushing.

I have yet to try pressure bleeding, (air only) and have been happy with vacuum bleeding, once the kinks were worked out.

chris