Brakes feel hard after leaving from parked position

I've had this once or twice and I've noticed that it seems to occur if I start the car with my foot already hard on the brake. When I try to repeat it it doesn't do it.

The brake pedal feels rock hard and there is no more 'give' in it if you press it. Will keep an eye on it and see if I can work out what is causing it. It has never happened when driving - just when I first start the car up (put it in Ready mode).

 

Sounds normal to me. In my other cars, if you have your foot fully down on the break when you turn off the igntion, and then pump the pedal it firms up to rock solid. if you continue to apply pressure and restarted the car the pedal softens up and goes down.

Maybe it's something to do with the first part of pedal travel being regen braking, and the remainder friction braking, and also something to do with the way the vacuum pump for the brakes work (duty cycle etc). i certainly wouldn't worry about it unless it happens at speeds above 10mph. slower than that and you can easily apply enough pressure to stop the car.

 

There is no "first part for regen, second part for friction" with the Prius. In normal operation the brake pedal is only a control input, with pressure sensors telling the ECU how much braking the driver desires. The whole regen vs. friction decision is done by the ECU.

Likewise there is no vacuum pump for the brakes. Perhaps you are thinking of the brake accumulator pump, which is a pressure pump for the hydraulic system.

Tom

 

"..only a control input.."

Not quite the full story...

When regen braking, regen ONLY, is being used the ABS/TC brake fluid pressure manifolds BLOCK ALL pressure to the brake calipers. In order to give the brake pedal the proper, standard, "feel" there is a spring loaded chamber, "compression cylinder", in the brake system that is opened to simulate the feel of brake pressure flowing into the calipers.

Should it happen, by happenstance, that the brake simulator chamber not be opened AND only regen is being used the brake pedal would likely feel as you describe.

Might be a "corner case" that the design engineers overlooked.

 

Yes, obviously, something like this is happening, or otherwise the brake pedal would never feel hard. My point was that the driver never directly controls brake operation for normally operating Prius brakes. I was trying to avoid explaining the intricacies of the stroke simulator and associated valving.

Tom

 

The brake pedal, "boosted brake pedal", is the direct source of hydraulic pressure/braking at all times, is it not...?

Knowledge, in depth, leads to understanding ;<)

 

I'm not sure I understand your question. Normally the brake pedal only pushes against a stroke simulator, which is really nothing more than a piston with a spring. The driver presses on the brake pedal, which creates hydraulic pressure, which moves the stroke simulator piston against the spring. The hydraulic fluid in this part of the system is completely isolated from the friction brakes unless there is a failure in the power brake system.

Pressure sensors in the brake line tell the ECU how hard the driver is pressing on the brake pedal. Based on this input, the ECU decides how much regeneration and friction braking to apply. Friction braking is applied by opening valves to allow pressure from the brake accumulator pump to drive the pistons in the friction brakes. The brake pedal and master cylinder remain isolated from the actual friction brakes. It's all brake-by-wire when everything is working in a normal fashion.

Tom

 

Always dangerous for me to comment on mechanical things, but here goes:

1. For the benefit of OP, it should be pointed out that this was a widely reported complaint when the car came out last year, and can be found in numerous threads, including the sticky thread on commonly reported complaints. The complaints seemed to go down over time, with some people reporting that it seemed to happen less frequently as the car became broken in. I haven't seen many complaints about it lately. Not sure is this is because it's happening less, or because it's such a widely know quirk now. I personally only experienced it once, after being parked on a slight incline and setting the emergency brake. Perhaps some change in software/manufacturing has also contributed to the fewer recent complaints.

2. There have also been widespread instances of people inadvertently invoking the hill-hold feature, by pressing the brakes too hard at a stop or when first starting the car. I did this myself a few times when I first got the car, and have witnessed other people make this mistake when first getting the feel of the car (most notably, an experienced hypermiler who was driving the 2010 for the first time). I personally always suspected a connection between the hard brakes complaint and the hill-hold feature. I suppose a combination of circumstances not anticipated by the software could also be causing the problem, as suggested by wwest40; or perhaps a slightly slow startup sequence? It doesn't seem to happen to people who don't try to rush their start-ups (same point applies to people who report sometimes not being able to put the car in gear on start-up).

 

It is quite possible that the heavy feeling of the brake pedal comes from
the charge in the brake system backup source capacitors being
discharged before the car reboots on startup: (Click to enlarge.)



I suspect that the Gen III braking system is essentially the same as
what's in the Gen II. The above is an extract from this good first level
explanation of the Gen II braking system.

 

Maybe I'm mistaken but I thought than once frictional braking is needed the solenoid to the "simulator" is opened and normal, boosted foot pressure takes over. On the other hand I've only really studied the 2010 technical manuals so that design might even be unique to the 2010.

 

I suspect with #2 you have hit the nail squarely on the head. When starting a car I always have my foot on the brake, but sometimes in order to trip the park release solenoid I must then press a bit harder on the brake pedal.

I understand that it is this "pressing harder" on the brake pedal after the car has come to a full stop, or is already stopped, that triggers, activates the hill-hold feature.

 

The Gen III system is substantially different than the Gen II. The same general ideas apply, but they differ in the implementation.

At least with the Gen II system, the solenoid valves only connect the brakes to the master cylinder when power is lost. Otherwise the system brakes entirely by wire. I believe this is still true with the Gen III, although the action at fail-over has been modified.

Tom

 

Also experienced this, but again much like you it goes away after another pump or so. Very strange and scared the crap out of me the first three times...

 

Wasn't trying to give you a hard time, just letting you know. Some people describe it as the brakes grabbing in reverse issue. Others have confused it with the now fixed ABS issue. Here's a link:

http://priuschat.com/forums/gen-iii-2010-prius-main-forum/65875-braking-in-reverse.html

You might also be interested in this thread:

http://priuschat.com/forums/gen-iii...2-summary-problems-reported-prius-2010-a.html