Okay. [cracks knuckles]
There's only two things it can be: Either the master cylinder, or the proportioning valve. Both are almost new, and both were working just fine a week ago. So as neither yelling at it or writing about it on the internet seems to have solved my issue, it's time to jump back in and get my hands dirty.
I decided to tackle the master cylinder first, as it's easiest to get to and dismantle. Thanks to rust-free stainless fasteners, it popped right off.
Drained and on the bench, we see it's unlikely it got clogged with rust or other gunk- the damn thing is only three years old.
By the way: Don't let anybody tell you- not even me, 'cause I have
- that POR-15 is immune to brake fluid. I'm here to tell ya it ain't.
The can says it is, the reviews said it was, I've had other hotrodders say it was... but it ain't. Basically everything on the back third of the master cylinder could be peeled off in big flakes. 'Coupla you Guilders told me about that back when I was putting this thing together, and nope, I was still goin' by what the box said.
Anyway, a master cylinder is easy to rebuild- or in this case, just inspect and clean. A big snap ring out the back, and then there's two pistons and a spring. That's it.
I cleaned everything off, inspected the seals closely, probed the ports for anything that might be jammed in there, wiped everything down, lubed the seals with some fresh DOT4, and put 'er back together.
I found nothing
to indicate a problem with the master cylinder. The seals were in top shape, there was zero gunk that could clog anything, there was no rust, no mechanical damage, and no missing parts.
So, I refilled the reservoirs and "bench bled" it.
This is basically just a process that recirculates fluid through the pistons, to drive out all the air. Regular bleeding on the car doesn't do it very well- it actually took me about ten minutes of cycling, waiting, cycling, waiting, topping off, and cycling some more, before I finally stopped getting air bubbles out of the ports.
To reinstall it, I just bent the tubes back up out of the way, and latched the cap in place. Then took it out to the car and bolted 'er back on.
So the master cylinder is almost certainly not the problem. That leaves the proportioning valve, and the obvious bit on the also-nearly-new P-valve is the "switch" or "differential" piston. This is a spool in the center of the P-valve that balances between the front half and rear half of the brake system. If a leak occurs on one end or the other, the differential in pressure shunts this piston to the weak side, closing it off and supposedly preventing any further fluid loss (at least at the master cylinder.)
That's basically what my system has been doing, with the fronts bleeding normally and the rears being unable to be bled at all. But we'd also tried half a dozen times to "reset" the piston- if it was indeed the culprit- but all to no effect. And I'd been able to draw fluid through with the MightyVac, so I'd kind of been leaning towards the master cylinder.
Okay, so the MC has (very probably) been eliminated as the offending part, which leaves the P-valve. And the P-valve, being nearly new, is unlikely to have spontaneously failed, which points towards this issue being something it's supposed
to be doing.
Something I should have tried early yesterday (isn't it interesting how obvious things seem in retrospect?
) is to put a multimeter on the bias switch. This is a later-model P-valve, and the switch is different than that of the plug on the car, so I don't have a dash light. (Which, in retrospect, if I did have it, it would likely have sped up the diagnosis by about 20 hours.
And I'm sure some of you smug bastards know exactly what I found, right?
Yup. Full continuity- the switch was "on", meaning the piston was indeed biased to one side.There's the problem that's been bugging me all weekend. (At least, the parts of the weekend I was able to work on the car.
Unfortunately, that was all the time I had today to fool with it, and duty called me elsewhere. Hopefully I'll have a chance to fool with it some more on Monday or Tuesday.