Front brakes stuck after fitting new pads

Interesting ... that sounds like actual binding, not just lazy return from the seals. We'll know more when you get them apart.

There was another PriusChat poster in the last year or so (as I recall) who was surprised to find the phenolic piston had swollen and become a tight fit. Surprised because that's not expected to happen; the manufacturers have worked hard at phenolic resins that don't do that. If I remember right, he had a long steep hill on his commute and thought unusual heat might have had something to do with it. New pistons fixed it for him. They're not expensive, but have to be ordered separately if you need them; the rubber kit doesn't include them.

By the way, if they are tightly binding and hard to remove, it's best to use fluid pressure to push them out (and give them something soft to push into)—they can be cracked if shot out against a hard surface, as compressed air might.

-Chap

 

Miles are nearly meaningless for calipers and caliper pistons, most likely moisture got behind the boots and once the pistons were pressed back in, they now bind in the caliper bore(s) due to rust. Definitely would not discount hardened caliper piston seals as 13 years old, but they come in the kits.

 

Can you get rebuilt calipers easily? The simplest route?

 

You can walk into any of the {Auto Zone, Pep Boys, Murray's, Advance, ...} sorts of stores and they will sell you something purporting to be a compatible rebuilt caliper on the spot. In my earlier years, I did consider that easiest, and did it routinely. In many cases, the caliper body itself will be a genuine OEM casting from a caliper that got replaced with one of these remans, then found itself going back as a core return to the same remanufacturer.

It was my experience a couple years ago that changed my thinking on that ... I had a 14-year-old original Toyota caliper that had gotten sticky because of a nick in the rubber boot, and I wanted easy, so I bought a parts-store-special rebuild and slapped it on. It had obviously been a real Toyota caliper once, though it now had obviously different piston and other hardware, and a different finish that they called "exclusive rustproof", which meant it was turning to red powder by a month after I put it on. The remanufacturing process had involved bead-blasting it within inches of its life (even the originally flat machined mounting surfaces were now blast textured), and, oh yes, right out of the box it had zero piston return.

That was when I discovered Toyota itself offers certain parts as reman (check for the part number with -84 suffix, they were available for my Gen 1 anyway). It was about twice the price of the cheapo one, and the quality of remanufacturing was unmistakably like night and day. The machined places were still flat, it was refinished to Toyota spec and did not start rusting right away, and the piston and all hardware and grease were as Toyota spec'd (which also means it would be rebuildable in the future using Toyota rebuild kits).

The best part was, I discovered that before I had returned my original caliper for the core refund from the cheapo purchase. I just let the third-party rebuilder keep my core charge, and sent my original Toyota caliper back to T so it could be well remanufactured instead of sloppily.

Nowadays I just keep on hand one rubber kit each for front and rear, and figure I'll do quick caliper checks when I rotate tires, I ought to catch any problem in time to easily rebuild on the spot if need be and be done with it, even if the stores are closed.

My most recent check showed flying colors for everything, except I changed out the fitting kit on the rears because they were binding some. A little bit of attention once/twice a year seems to be enough to keep everything in top shape.

-Chap

 

You can, but it is simple as a DIY project and about $10/caliper to do. At least you know what you have that way.

 

Yeah this is the first car I've seen with plastic caliper pistons in the front. You need to be careful pushing them back, no point-loading, have uniform contact around the rim: someone here managed to crack one IIRC.

If you cannot put a piston back in without seal, that could be very well be the core issue.

 

Prius went from cast-iron front calipers with steel pistons to aluminum with phenolic pistons between Gen 1 and Gen 2 in 2004 (2004 New Car Features Manual, page CH-31).

Phenolic caliper pistons aren't especially rare on modern cars; it's easy to find articles about them. They're lighter, they don't rust, and they are less heat-conductive, which means less heat carried back into the brake fluid.

There are articles that do refer to swelling as a possible issue with phenolics. I remember seeing one saying the problem was worse with the earliest phenolic pistons (many years ago now), and the manufacturers have worked successfully on solving it.

I have to say yours is only the second case I've read on PriusChat of the piston swelling. It would be interesting to try to figure out what operating conditions seem to make it more likely.

Here's one article studying the issue (starting from an observation that calipers of the same type, with phenolic pistons, used in delivery trucks, RVs, wreckers, etc., were having swelling problems in the RVs but not the other vehicles).
https://static.nhtsa.gov/odi/inv/2007/INRP-EA07016-35282.pdf

-Chap

 

Did you happen to look at the NHTSA study linked above? I thought it was interesting that they didn't even write a 'conclusions' section at the end, but maybe you don't have to when the box plots are that dramatic.

A couple telltale points: in the group of pistons whose diameters increased the most ... so did their weights.

Also, in the group whose environmental chamber lost humidity toward the end of the test because of a gasket leak, the diameters and weights decreased toward the end.

Also intriguing was the question that teed off the whole study: why the swelling in RVs but not delivery trucks? Delivery trucks carry heavy loads and stop a lot; their brakes presumably get hot.

-Chap

 

Maybe it's the difference in training and professionalism of the drivers. Also, some trucks have drive line retarders and engine compression (Jacobs) brakes.

Some of the trucks I drove had electro-magnetic retarders, so the "B" function on the Prius just seems natural. I keep trying to shift it into other retarder positions.

 

Way more likely just because of use the brakes including the brake caliber piston get hot and evaporate the moisture. RVs sit long time without use compared to delivery trucks that are used all the time.

In the case of Harryarcos Prius it’s way more likely that swelling was there before but it only really became a problem after the piston was moved back. It getting stuck because it was swollen caused brake to heat up.

 

Regular brake fluid changes would help then?

Toyota Canada is saying 3 years or 48,000 kms (30,000 miles).

 

Maybe, maybe not. Less than half of the piston surface is in contact with brake fluid. Other area is just in the atmospheric air. Some of the area is other area is under dust boot but that isn’t really air tight.

Toyota Finland says to replace brake fluid every other year on all of their cars. Actually basically every importer here tells to replace the brake fluid every other year.

 

Maybe your country's regulations? Here I know Honda Canada also: they say tri-yearly, regardless of mileage.

 

No it’s not a regulation as there are some cars that say to replace it every 4 years. And many where you first replace it after 3 years.

 

Good to know, thanks.

Will Toyota sell you a rebuilt caliper? At this juncture it might be most expedient, albeit more expensive.