Grabby Brakes, Only When Backing + Damp

Any noises?

A rough / grind-y noise typically indicates some rust has formed on the rotor surfaces. It often happens if the car sits for a while in damp weather, and also makes the brakes grabby.

Making a few normal stops in neutral will usually be enough to rub the rust off. When the rough / grind-y sound goes away, the grabbiness usually does too.


A hiss / puff / whoosh noise, just as the friction brake apply or just as regen transitions to friction, generally means there are bubbles in the brake fluid. The actuator opens valves to send pressurized fluid to the brakes, but instead of the pressure building immediately the way it does with incompressible fluid and no bubbles, the pressure lags because the bubbles compress and the actuator has to whoosh more fluid into the lines before the target pressure is hit.

That can make the brakes grabby because the ECU doesn't have as fine control over the pressure as it normally has; it has to move larger amounts of fluid and wait for the bubbles to squish and the pressure to catch up, and it may overshoot in the process.

Going through the full-system bleed procedure with a suitable scan tool can get those bubbles out and resolve that.

Finally, if you're sure you don't have either of those acute problems, but the braking force is just never as nicely modulated as it could be, there is a "linear solenoid offset" that you can have the system clear and re-learn. You just stand there for a minute or so and hear the car apply and release the brakes with different amounts of force, and it learns the valve openings it needs to achieve different levels of force.

If the system has gone substantial years or miles since the last time those levels were learned, just repeating that process might make the brakes noticeably better modulated.

As for "get a brake job", I don't really believe in "a brake job", so much as just getting the wheels off and inspecting the brakes, and dealing with any issues you see. You can sometimes catch small issues and fix them on the spot with no new parts.

 

There is also a "When not using the Techstream" version of the linear solenoid procedure, described in the repair manual.

Toyota Service Information and Where To Find It | PriusChat

Your mention of "wet / damp days" really puts me in mind of rotor rusting, even if you are not noticing a rough or grinding sound.

 

Not sure this is related, since it happens very rarely, and weather doesn't seem to be a factor. but once-in-a-blue-moon I'll get an extremely touchy brake behavior, typically just after start-up, and in reverse. Doesn't last long either.

First time I noticed it was backing up a bit in a parallel parking spot, in preparation to drive out: started the car, began to back up, touched the brakes and came to an abrupt stop. For a moment I thought I'd misjudged and tapped the car behind, it was so abrupt.

The behavior doesn't last long, gets back to normal, usually once I start rolling forward.