I bought a new mc under the assumption that, after 140k let's take an old part out of the equation. I'll bench bleed, install and update. Probs sunday
Are you pushing the pedal to the floor? If so, you may have damaged the seal(s) in the MC.
My experience: '08 Tahoe, ~150K miles. Bleeding brakes just as I've done countless times on many vehicles. RR, LR, RF and when bleeding the final corner, the LF, the pedal went soft and would not firm up. Nothing but bubbles in the hose. I know I didn't let the MC go dry. After starting all over again with a soft pedal no mater what, I got a new MC. Bench bled it, installed it and bled the system all over again. Been great ever since.
Turns out, the bore of the MC wears where the piston is routinely cycled through the miles and miles of normal driving. This leaves the far end of the bore, where the piston never goes, unworn and slightly tighter. When you floor the pedal during bleeding, you're repeatedly jamming the piston, with its aged and weakened seals, into a tighter bore. I'm sure it's more common for a failure to
not happen from this, but there are instances. So, yeah, drop a brick or 2x4 or something to block full pedal travel.
Also, I've never stopped during a brake job, put the wheels on, drove to a grassy field or gravel pit to cycle the ABS, drove home, got back under the car then bled the [now hot] brakes again. When you turn the key on and all the cluster lights come on for the self-checks, the ABS solenoids are also cycled. I hang the hose and bottle on the corner I'm bleeding, crack the bleeder and test the pedal. It just needs to "leak down" under firm foot pressure and not be a totally loose pedal. You want it to take about three seconds to make a full stroke. If you still have to really press on it, the bleeder is too tight. Give it an ~1/16 turn or so and test the pedal again. To bleed, slowly but steadily push the pedal a few times to your pedal block. Then, just as you're beginning another stroke, turn the key to on (not start) while maintaining your slow stroke. Give it a stroke or two, turn the key off, a few more strokes, key on, few strokes, etc. Check the hose for bubbles and top off the MC reservoir. I suggest you borrow a friend to watch the hose or even just use their phone if they're unavailable to hang out. Set the phone up to watch the hose and Facetime/video chat it while you watch it with your phone from the driver seat. When you see a few strokes cycles with no bubbles, go close the bleeder screw, set up and repeat at the next corner.
If you have a power bleeder, use it as normal (but keep an eye on that MC reservoir!) but have someone cycle the key a few times while you're operating the bleeder at each corner.
