Strange surging during very slow speed braking, stopped

I think you need to clean your throttle body

 

And sensor clean off too MAF map etc

 

Does it feel like the engine power is surging (which the two previous replies seem to be getting at), or like the braking force is not holding steady?

It can happen that one of these older Prii can develop an internal leak in the brake system, where the brake fluid pressure bleeds off slowly. If you are applying the brakes while this happens, there can be a cycle where the pressure at the brakes slowly decreases, and the system notices and opens valves to bring the brake pressure back up, and it slowly decreases again. Sometimes if the environment is not too noisy you can hear soft clicks near the firewall when the system boosts the pressure up again (which would coincide with feeling more forceful braking again). In some cases, the clicks might come with a sort of quack or bark noise.

Also, sometimes, you can catch the same thing happening with the car not moving at all, if you just hold the brake pedal pressed, and listen for a sort of repeating valve-click sound near the firewall every couple seconds, with or without a little quack.

 

have this in two 08 now . accumulator bleed down whilst in operation.

 

I think he first needs to check his rotors for rust.

Mine does something similar to that if it sits out in the rain or high humidity for a couple of days without being used.
Where he lives, that could be the situation almost EVERY day.

A couple of "firm" stops from speed or intentionally dragging the brakes at cruising speed eliminates it.

It feels the way it does because the mechanical brakes don't actually make contact if
the regen braking is slowing down enough. The brakes don't actually engage
every time you push lightly on the pedal.

 

There is a certain pedal pressure above which that is true. You can find it by watching the display page with the little car and the arrows, and finding how hard you have to press the pedal to make the arrow to the electric motor go away.

As soon as you're not pressing quite that hard, it sends power to the motor again, all part of the game to fool you into thinking it's your grandfather's automatic transmission wanting to creep forward.

 

Pumping the brake enough will make anybody's booster pump run. (It's the booster pump you're hearing, which is separate from the booster. The booster is silent.)

The better question would be, if you just hold the brake applied, no pumping, do you hear a repeated clicking or thumping sound (with maybe also the sound of the booster pump again, briefly, after every so many clicks/thumps)?

 

Probably time to start budgeting for an actuator.