Rear brakes locking up in cold, rainy weather.

Do they salt your roads out there in Ohio?

 

Hi Rob,

Three thoughts:

1. Might the driver-side rear brake shoes be binding against the backing plate? Each brake shoe contacts the backing plate at three points which are lubricated with high-temp grease. Perhaps those points have become rusty.
2. The shoe return spring located below the brake wheel cylinder that pulls the driver's side brake shoes together may have become weak.
3. What does the driver-side brake drum look like? Any discoloration or cracking inside or outside? Suggest you compare it to the passenger-side brake drum.
4. According to the repair manual, the correct adjustment for the rear adjuster is 8 notches after the drum locks.

Good luck,

Patrick Wong

 

Whatever you do, do NOT attempt to bleed the brakes yourself. The Prius has a sophisticated electric assist braking system that could be rendered inoperable if you attempt to bleed them yourself. Please read the attached Adobe pdf's which have the correct procedure, and the brake diagram

 

I recognize that Prius has a highly complex brake system and that the THHT or the new service laptop is required to bleed air out of the system, allowing the various solenoids to be opened & closed in correct sequence, etc. However, I have successfully "flushed" new brake fluid through my 2001's brake system when it had ~60K miles without special tools.

The key is to not allow any air to enter the system north of the brake wheel cylinders. If air enters, then your only recourse is to tow the Prius to your local friendly dealer for the official brake bleeding process.

My procedure was to use a rubber squeeze bulb to suck out as much of the old fluid from the reservoir as possible. Then I filled the reservoir with new fluid. Starting with the rear and then moving to the front, I attached a vinyl hose to each wheel cylinder bleed valve in turn, with the other end leading to a small container. I had my wife turn the Prius to IG-ON, then depress the brake pedal a tiny amount, holding it steady. I cracked open the bleed valve and a strong flow of fluid resulted. Close bleed valve, release brake pedal, refill brake fluid reservoir, repeat, etc. I went through two small containers of Toyota DOT 3 brake fluid, and felt the above process resulted in replacing 80-90% of the old fluid.

I'm planning a similar process with my 2004 when it gets to 60K miles (currently at 46K miles).

Patrick Wong

 

I would strongly caution you against doing that. It's your car, do what you feel is best, but there are key differences in the +2004 Prius braking system

Toyota uses electric assist brakes in more of their new vehicles, but the majority of them only require the THHT or Panasonic ToughBook if you have air in the master cylinder. My FJ is one such Toyota with electric assist brakes that can still be flushed the "old fashioned way"

Note that with my FJ, if the master cylinder should run dry, the THHT or Panasonic laptop is needed to command the solenoids

 

I received my Bentley workshop manual yesterday and was suprised to see that the US models have rear drum brakes. The one here in Oz have discs with a small concentric drum for the parking brake only.

AB-S

 

I know this is an old thread. But I am having same issue arising for the first time today. In my case, RR brake is locking up when I am almost stopped (best guess: at about 3 or 4 mph). And, yes it is occurring on a wet, drizzling 40F day. I am on the road now. I expect that I will tear down the brakes tomorrow and likely replace shoes and hardware.

Any additional suggestions would be greatly appreciated.

 

I would often do a "light" maintenance, with rear drums, might be all you need. Basically remove the drum* and blow the dust out (don't breathe), then remove the "shoe hold down spring clips".

At this point everything's still together, but the shoes are loose, and you can pull them away from the backing plate and carefully apply a little dab of grease at the 3 contact points (per shoe) indicated in the pic. Use something like a knitting needle, and be very careful to not get it on the shoe. I've used some high-temp bearing grease I've had for decades, using it up.

* If the drum is stubborn, you can screw two m8x1.25 bolts by about 1" long minimum, fully threaded) into the threaded holes on the front of the drum, there for just that purpose. Just go back and forth, screwing them in. Might need to back them right out and blow the rust out once or twice. Eventually you should be able to pop the drum loose.

Do remember to have the parking brake released, lol.



Note too: it's a good idea to disconnect the 12 volt neg cable before starting, and when everything is reassembled, pump the brake pedal mulltiple times, firm it up, before reconnecting the cable.

Full Repair Manual excerpt attached.

 

This happens to me as well. Not that I attributed it to rear brakes. Brakes when wet in general tend to grab at low speeds and the car stops suddenly from 3-4 MPH to zero with a jolt.

My take on it to attribute it to general brake behavior. My take is that on any car wet brakes would tend to grab. However if you are at a reasonable speed brakes get opportunity to dry out because of frictional heating. In case of Prius, most of that is regenerative braking in usual situations. Thus when brakes come into picture, those are still wet and thus grab. Just my theory.

 

Today is a bright and sunny day with a high of about 55F today here in northern New Jersey. And, yesterday's brake issue has self-resolved about 95% (IOW, I had two jolts today upon brake application during the whole day when driving (again this condition was occurring on deceleration at about 4 to 5 mph).

Just got done with the mobile mechanic. The mobile mechanic replaced the rear brake shoes (including lube at appropriate points), installation of new hardware kit and brake drum (one for right side that was locking up). Brake pedal is now high and tight (just as you would expect). Additionally, no more jolts when decelerating/braking at 4 to 5 mph.



Parts were obtained from Advance Auto as follows:



Wearever
Brake Shoes - New - Rear
Part No. S832
Warranty: LIMITED LIFETIME REPLACEMENT
Discount GL44BX -$8.75
QTY= 1

$34.99
Advance Auto Parts - Store 5182

==========================

Carquest Wearever
Brake Drum - Rear
Part No. YH140753
Warranty: 2 YR REPLACEMENT IF DEFECTIVE
Discount GL44BX -$10.99
QTY= 1

$43.99

Advance Auto Parts - Store 5182
=====================================

Wearever
Drum Brake Hardware Combi-Kit
Part No. 17262
Warranty: 1 YR REPLACEMENT IF DEFECTIVE
Discount GL44BX -$4.75
QTY= 1

$18.99
Advance Auto Parts - Store 5182
===========================================

Wearever
DOT 3 Premium Synthetic Brake Fluid, 12 oz.
Part No. W20013
Warranty: 30 DAY REPLACEMENT IF DEFECTIVE
Discount GL44BX -$1.00
QTY= 1

$3.99
Advance Auto Parts - Store 5182
===========================
Product Subtotal: $101.96
Discounts: -$25.49
Shipping Charges: $0.00
Shipping Discount: $0.00
Core Charges: $0.00
Tax: $2.53
Order Total: $79.00*


*- total price after 25% discount and sales tax (at 3 5/16% sales tax for Urban Enterprise Zone).
Used Advance Discount Code ("GL44BX") in order to obtain 25% discount. I was required to spend $100 in order to obtain the 25% discount (reason that I purchased the brake fluid was to put me over the $100 minimum).