Hey fellas,
Scratching my head with this one. Was driving home the other day and the brake pedal started creeping towards the floor with every brake application. Eventually the dashboard brake light came on and the pedal started going to the floor. Researching this it seems the centering valve/proportioning valve closed off one of the two circuits. That's all fun and good, I cracked the front and rear bleeders to see which one got closed off and it was the rear, no pressure at all when pressing the brake with the bleeder open. I look around for any obvious signs of leaking as I just changed the flexible hoses two months ago, very likely one could have come loose. No signs of an obvious leak so I set off to try to get fluid moving on the rear circuit so I can spot the leak. I pop open a front bleeder and leave the rear bleeders closed and pump the pedal a few times. The brake light does not go off and the centering valve does not return to it's neutral position. I thought it might be a bad master cylinder considering it's 26 years old at this point so I slapped a new one on. I'm not ready to rule out it's still a bad master cylinder as just because it's new doesn't mean it's good. Especially lately and especially since it's a Dorman.

So my question is, how can I get the centering valve to open back up so I can troubleshoot my low pressure issue? I've cracked the front lines at the actual valve on the ABS modulator and still couldn't get the rear circuit to pop it back.

Picture for reference.

 

I could be wrong, but I believe that the only way to reset the locked-out ABS valve is using a high-end scan tool that has access to ABS functions and can cycle the valves.

 

I tried that, but was unable to get it to pop back. I eventually cracked the front circuit at the master cylinder and pumped the pedal a few times and it reset. After bleeding the entire system (including the ABS system automated bleed process with a scan tool) I have full braking again. I don't have a leak in the rear so I'm not sure what caused the original fault. My thinking is I still had an air bubble on the rear circuit after changing my soft lines back in November and it made it's way to the proportioning valve and caused a low pressure event.