So an update from over the weekend. I had a long talk on Friday afternoon with the lead mechanic at Atlantic British. His assessment was as follows.
1) ABS air - Bleeding the ABS wouldn't be the issue. Any air introduced into the module through the standard bleeding of the bad master cylinder could also be removed with standard bleeding, which I've done in spades. If there happened to be air in the system that could only be removed by activating the system, then it would only present when the system activated, not consistently as is my issue. Additionally, air in the system in general would not present with a sinking pedal, but one that requires pumping to firm up (we noted this earlier in the thread). If the ABS is the issue, it would be an internal hydraulic leak between chambers or one presenting where I'm not seeing it. In either case, the unit would need to be replaced, as it's non serviceable.
2) Hydraulic - This is where he felt the root cause would be. Without visible leaks, he believed there were 4 possible explanations. 1) Hydraulic expansion or minimal leak (not yet visible) in the rubber system parts: caliper piston seals or rubber brake lines. 2) Pass through issue of the internal seals in the master cylinder 3) Seal leak internal to the brake servo 4) Internal leak between chambers of the ABS module.
So, here is the action plan and the results to this point.
1) Isolate the MC and servo from the rest of the brake system and test. I pulled the two brake lines and plugged with two bolts and o-rings to allow them to seat. With this set-up the pedal was completely firm and did not descend. So, the MC and servo should be officially ruled out and the issue further down stream.
2) I've ordered new seals for the remaining two calipers which I hadn't repaired yet, along with stainless steel brake lines. If figure those would be the logical next step. Simply because even if they don't solve the issue, it's still a good idea and not wasted money.
3) If the problem persists following that, it would seem I'm looking at the ABS.
Thanks so much for all the continued responses. They're really serving to keep my though process focused and moving in the right direction. I'll keep everyone updated.
1) ABS air - Bleeding the ABS wouldn't be the issue. Any air introduced into the module through the standard bleeding of the bad master cylinder could also be removed with standard bleeding, which I've done in spades. If there happened to be air in the system that could only be removed by activating the system, then it would only present when the system activated, not consistently as is my issue. Additionally, air in the system in general would not present with a sinking pedal, but one that requires pumping to firm up (we noted this earlier in the thread). If the ABS is the issue, it would be an internal hydraulic leak between chambers or one presenting where I'm not seeing it. In either case, the unit would need to be replaced, as it's non serviceable.
2) Hydraulic - This is where he felt the root cause would be. Without visible leaks, he believed there were 4 possible explanations. 1) Hydraulic expansion or minimal leak (not yet visible) in the rubber system parts: caliper piston seals or rubber brake lines. 2) Pass through issue of the internal seals in the master cylinder 3) Seal leak internal to the brake servo 4) Internal leak between chambers of the ABS module.
So, here is the action plan and the results to this point.
1) Isolate the MC and servo from the rest of the brake system and test. I pulled the two brake lines and plugged with two bolts and o-rings to allow them to seat. With this set-up the pedal was completely firm and did not descend. So, the MC and servo should be officially ruled out and the issue further down stream.
2) I've ordered new seals for the remaining two calipers which I hadn't repaired yet, along with stainless steel brake lines. If figure those would be the logical next step. Simply because even if they don't solve the issue, it's still a good idea and not wasted money.
3) If the problem persists following that, it would seem I'm looking at the ABS.
Thanks so much for all the continued responses. They're really serving to keep my though process focused and moving in the right direction. I'll keep everyone updated.