Hi Mark, great to see you post this, though I will say that it has me baffled as to how the brake system you have videoed holds that amount of vacuum. All of the directional cup seals in the system are designed to balloon out under pressure (MC, caliper piston seals). I'm very surprised to see that you have managed to draw that amount of vacuum and have it hold. Apparently the GM OEM cup seals are tight enough within their respective bores to also allow this strong of a vacuum to also be applied and held with the components you are using. I can see where square cut caliper piston seals may hold a vacuum, but the cup seals in the MC? Hmmm... Remember how everybody always talked about a ten pound residual valve in the rear brake circuit on drum brake equipped cars to prevent the cup seals from allowing air to get drawn in? My personal testing has shown that rear drums work just fine without the residual pressure... I am now curious as to how aftermarket MC's and caliper pistons may react to having a vacuum drawn on the system.
Ford has circulated a PS system bleeding procedure over the year that uses vacuum to assist in drawing air out of the PS system over the years too. I have tried this a few times on various different year / make / models with both empty and filled systems with zero success - will not hold a vacuum. On a running, happy and healthy PS system, my occasional previous testing with vacuum applied (while the engine is running) has actually "pissed off" the power steering system by drawing air in. I'm suspecting the front PS pump shaft seals (as they really aren't designed to hold any particular pressure), but haven't been able to prove it. Interesting that a high pressure hydraulic system is designed to hold pressure, but in my POWER STEERING
(not brake) testing, vacuum has always worked out to be a "pie in my face", so I no longer fool with it.
Interesting...
