Bad Master Cylinder After Brake Job?

[Lexus-Toyota-Scion Technician]

So I replaced the pads and rotors. I put in drilled rotors and PBR pads that had 200 miles on them. The shims were re-used. I applied brake lube between the shims. The anti squeal springs in the rear were replaced along with the pins. When I replaced the brakes, I pushed the pistons back into the caliper but I didn't crack the bleeders open

I have bled the brakes using a vac-ula, the scan tool, and manually (key on, assistant presses and holds pedal and waits for the actuator to build pressure). No air bubbles are coming out. I have tried bleeding 8 times or more.

Here is the problem. With a steady application of the brake pedal, I have a firm pedal. Near the end of my braking action, the pedal travels farther and the ABS actuator buzzes and it feels like the brakes give out.

I have two codes, C1223 and C1256. The first code is a generic code and the other is Low Pressure Accumulator...something. Basically a problem with either the accumulator, ABS motor, or the pressure switch.

A Master Diagnostic Technician believes that the issue is a faulty master cylinder and so does another technician. A new master cylinder is $1869 with a brake booster. A used MC is $325.

My question for you all is what do you think might be the problem? Has anyone run into this before?

[SRK]

I have no experience with the master cylinder on the GS, but I have some ideas on this problem.

When pushing the pistons back into the caliper it's possible that some dirt or debris was pushed into the master, and has lodged under a seal lip. When you describe a steady application as giving a firm pedal, and then a further application causing the pedal to drop, it sounds like a classic case of "bypassing".

I can't see how the seal could be damaged, just unable to conform to the wall of the cylinder. It's probably a lot of work to get the master cylinder unit out and onto the bench, but it may be an easy job to remove the primary and secondary pistons from the bore once that's done. Wash the parts in warm soapy water, dry carefully, lube with brake assembly fluid, and re-install. You can also inspect the edges of the primary seals to see if they have grooves or cuts in them.

If it were my car, I'd clean and re-install the cylinder parts and re-bleed before spending that king of money. Of course perhaps the hydraulic parts are not serviceable on this unit - I don't know. But if it can be done, it's worth a try.

Also build accumulator pressure, and shut the engine off. Apply the brakes with a varying load on the pedal. If at a certain pedal pressure the pedal drops noticeably, then one of the two primary seals is definitely at fault.