Brake lock-up after switching on.

I cannot be considered an expert on either the Gen III -- I drive an '08 --
or brake systems in general. Nonetheless I'll make a suggestion, purely
mechanical and simplistic in nature, which may be helpful.

Is there any possibility that your problem occurs during rainy periods?
At such times my '08 randomly shows evidence of the disc brake pads briefly
hanging up at start up… Its only a split second and is remedied by reapplication,
And takes only a few feet of moment in the driveway before it is gone.

Perhaps there is a rainwater/rotor corrosion connection for your issue.
Am I correct in thinking there is a disc brake caliper lubricant which might
free things up?

 

From what you just described it sounds like the part of the brake system that simulates the feel of a mechanical hydraulic brake system is being over pressurized. Since it was happening prior to the brake booster pump replacement it does not sound like this is the source of the problem.

 

The car should be able to withstand whatever force you can apply to the brake pedal, short of bending the pedal arm, and the resulting quick stop, without damage. Some cars have had the light switch come loose and shift out of correct position, but that is not the result of pressing the pedal too hard.

When this happens, have you put your full body weight, or at least 500 newtons / 50 kg-force / 112 pounds, into the brake pedal? If so, and you don't get useful braking effect, then this would be prima facie evidence of a problem or defect in the braking system in the U.S. At lesser pedal forces, the issue becomes murky because of legally allowed (in the U.S.) failure modes of the power brake boost.

Other threads have expressed hints of some transient brake failure that fit much of your description, excluding the previous hard braking and the light switch issue. I even experienced it in my 2010, but no critical mass of events and clues has yet emerged.

 

Are you seeing the red (fault) or yellow (warning) triangle light when this happens along with any message displayed? If so then a diagnostic code has been triggered. If not then no codes are being triggered. If codes are being triggered you need to get someone to read these and post on here.

A Scan Gauge can read standard Engine and Emission codes but not Prius Skid Control ECU codes. A custom XGauge will need to be configured to read these codes. You most likely will need to go to the dealer to have the Prius specific codes read.

 

One suggestion would be to capture a video of your foot on the brake and the car rolling forward. Take the video to your dealer. This has worked for other members.

 

The excessive hard brake pedal would indicate a malfunction in the brake system actuator. The loss of boost to the brakes should be considered a major malfunction. Personally, I wouldn't drive the car until this issue is completely resolved. The dealer should get Toyota directly involved again.

 

My concern would be that this malfunction in the brake system might happen somewhere besides your driveway. IMHO, you shouldn't have to drive your car in a state of paranoia. I applaud the efforts of the dealer but they shouldn't have replace anything until they properly pinpointed the problem. Letting you take the car without knowing specifically that they had the problem solved is reckless. If they couldn't figuer it out, they should have called in Toyota engineers to pour over the car while you where driving around in a loaner car.

 

How old is your starter battery? I just had mine replaced about 3 months ago, but the one they put in had a bad cell. I was experiencing a similar problem when starting my car. Brake pressure when starting the car was acting funny. I say go get that checked out and report back.