I finally got this car fixed. Thank you all for the help. There was a difficult variable thrown into the mix during this whole ordeal. I did access the ABS ECU under RH side of dash and verified master cylinder pressure sensor voltage readings were the same there as when I checked them at the pressure sensor itself. No issues there. I did end up replacing the master cylinder assembly (reservoir included which is key here) using a new Toyota part from the dealer. After this the C1246 code never reset but what did start setting was a C1301 for communication problem with the ABS translate module. I followed the Toyota procedure for diagnosing this code which is where I went wrong! It led me to remove the ABS translate ECU and test for communication at the harness which is coming from the ABS ECU. I did have communication but it looked nothing like the example waveform on the Toyota example in SI. It led me to replace the translate ECU. I ended up locating a used translate ECU & ABS ECU from a salvage yard for $35 ea. I installed them and no change. I scoped and the pattern looked identical to the original ones. To shorten this up and get to the fix I learned that the C1301 code was now setting because the brake fluid level sensor that came on the new OEM Toyota master cylinder was defective. I had 12V on the red/white wire and ground on the other. When I moved the float with a pocket screwdriver I could clearly see it switch from 12V to 0V and then 0V to 12V. This was the issue. It should have been switching between 6V and 0V because there's supposed to be a resistor across the brake fluid level switch. I flipped the original master cylinder reservoir over and measured resistance across the switch which read 1.9K ohms. I did the same with the new reservoir and it read OL. The new reservoir was defective causing the C1301 code which threw me off big time. I sure didn't expect to have a brand new Toyota part be defective. Out of desperation and not wanting to wait 5 more days for another potentially defective part I installed the original reservoir on my new master cylinder. I then cleared C1301 and it did not reset, brake system MIL lights went out, and test drove perfect with no codes resetting. Several lessons learned here. I'm sure glad to have this one fixed.

Jason Wieland
Technician/Owner
Ace Body And Motor
Des Moines, Iowa, USA