The rear brakes on my wagon have been squealing for a while now, and the right rear caliper was leaking fluid from the ebrake crank. Rather than bleeding the whole system twice I took the opportunity to gut the proportioning valve so the rear disc brakes can finally start doing their fair share of work, as the stock part was valved for having the old drums.
After doing this, the rear brakes were INSANELY loud. Like feeling the howling through the pedal, blow out your ear drums because of the resonance loud. I suspect this was due to the pads not bedding in properly from under-use from the prop valve. A new set of better quality pads and lubricated caliper slide pins later, and the car stops perfectly and silently.
The moral of this story: if you convert from rear drums to discs, make sure you adjust the proportioning to compensate, otherwise your fancy new rear brakes aren’t going to be doing much of anything at all.