Braking Issue

It sounds like something is causing a wheel to lock up and skid and the system is doing what it's supposed to do. I usually feel that when accelerating out of gravel--it's disconcerting when it happens.

Have you ever had the brakes serviced? In the UK, you probably have rear disc brakes. Here in N. America, I'd suspect the rear drum brakes first. You do not need a dealer to check the friction part of the brakes--any shop can do that.

You should check bushings, struts, and bearings too. Again, any shop can do that.

Unrelated, it's probably time to flush brake fluid if you haven't done it. Hydraulic system work is a dealer or Prius-specific mechanic job.

PS: Thank you for spelling "brakes" correctly. There have been a few threads lately calling them "breaks."

 

What is the tire tread depth and inflation pressure on all four wheels?

 

Your rotors can rust and you won't feel it until the brake transitions from regen to friction brakes. Try a few non-regen brakes to clean things up. From about 40 mph (64 kph) slip the car into neutral and brake to a stop. Do this about 3 times. The car can't regen in neutral so it must use the brakes. I find this useful after a rain or if the car has sat for a few days.