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WATER DAMAGE TO REAR CALIPERS under normal use '03 Forester xs


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Hello all. 2003 Forester XS. I installed purchased fresh rebuilt stock calipers from boxes 2 years ago, along with new pads and rotors, and a complete brake bleeding, here in the North East. I just had to rebuild them myself using new pistons and new rebuild kits with a light honing,

this last winter after water got into both caliper pistons apparently from by passing the outer piston seal. Really badly on the right rear.This caused stuck calipers, and completely separated the friction material  on 3 of the 4 rear brake pads from their backing plate on smooth rotors. One piston was very badly rusted

 inside, where it should have been protected from any water getting in. My guess is the sand/bead blasting on the rebuilt calipers cause a

compromised water seal from the start. Another is the design of the stock rear calipers seems to be not geared to keep water out well either.

In any event, I put a bead of copper high temp silicone around the new piston outer seal, on the rebuild, but that really does not hold well over time due to piston movement

and brake vibration. This seems ridiculous. Because, the calipers were ruined/stuck not from internal deterioration and lack of brake bleeding, but simply

 because water easily gets past the outer piston seal no matter. In any event, I may end up getting 2 piston STI calipers on there eventually.

But I am shocked at the poor design allowing water in, when this vehicle is not driven  off road, or abused. It is a 2003 Forester XS and has alloy wheels

which may contribute to more water exposure though.

I now believe that worn rear wheel bearings played a big role in causing excessive play in the hub, and allowing the brake rotor to move side to side

 ruining the brake pads, and subsequently causing the caliper pistons to shift and allow water in. Even though the seals were not torn.

Still, I think the rear caliper seal is not very effective in keeping water out from the inside of the caliper piston, and makes rebuilding or replacing

the rear calipers a much more frequent even than one would assume. I now also hose off the calipers in winter to keep ice and crud which traps moisture, off of them, but I am possibly eventually looking for a better caliper conversion like the 2 pot Subaru ones. Not for a performance increase mostly, but simply

 because they are possibly better designed to keep water out of the caliper pistons. And will not require rebuilding as often. I mean, if I am looking at a caliper rebuild every 1-2 years, or at least replacing the outer caliper seal, that seems excessive. Any thoughts anyone?



Edited by bakedpotatoechips99
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Is the car ever parked over grass?  That aids rust.

Are you having other significant rust related issues - exhaust?  Wondering if you're conditions are exceptional, beyond normal Northeast standards.

How many brake fluid flushes have been done since you've owned it?


I would suspect something is compromising the fluid and inducing rust internally.
Or the piston boot seals aren't of good quality.  I'd aim for making sure whatever you install (rebuilt?) has Subaru seals/dust boots.


Subaru calipers are robust and routinely last the life of the vehicle, even in the northeast rust prone areas.

That being said - of course rust is also a problem around here.


Oddly if rust is an issue you haven't mentioned the super common guide pin issues - I have no idea how many rusty guide pins I've repaired in newer Subarus - it's common.  Rusted pistons happen, but not nearly as common as guide pins.

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  • 6 months later...

 Car was not washed. This is likely a large contributing factor to salt staying around the caliper, intruding past the piston seal etc. The fluid was completely flushed in all 4 calipers 

  when the rebuilt calipers were installed. And new brake fluid was used. No leaks in lines. There is a new mix of road salt with magnesium chloride I hear that is highly corrosive, and the State of CT is actually being sued

 by some owners of cars whose engine cradles rusted all the way through, and dropping their engines out. (not Subau's though). And, apparently there are dangers of semi trucks brakes corroding much faster as a result of this,

 posing risks to the general public. Obviously this new salt mix prevents ice on roads, but the costs to vehicles and bridges etc., caused by increased corrosion is being weighed.

 But back to the '03 Forester rear calipers. The single rear seal on each piston seem inadequate to seal out water. And no, they were not genuine Subaru parts. Rebuilt, so likely more porous from bead blasting by rebuilder

 prior to rebuild/re-manufacture of calipers, prior to my purchase and install, and subsequent rusting completely shut/stuck 2 years later. 

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I would consider the possibility this was an outlier event, a bad part/low grade aftermarket part, fluid contamination, etc.


Subaru calipers routinely last the life of the vehicle with very few and rare failures.  Google and forums like this show how rare brake issues are.

Many brake calipers are only replaced due to stuck slides, not failed caliper pistons.


I'd install used Subaru calipers and expect 100,000 more trouble free miles.


If the rust is that bad you'll be replacing exhaust, fenders, rear alignment trailing arm bolts...etc. Rear brake pistons/calipers are not the first parts to rust out and certainly won't be the only thing.

I've already replaced all of those parts on mine due to rust - and i have the original brakes at almost 250,000 miles now.

That's the norm, there's nothing special about my car.


So if you're just having caliper issues - I'd keep this simple and not try and blame it on a poor design.


Get a known robust part - Subaru OEM - used or new.

Replace any hardware not included - boots, retaining clips, pins.

Use SilGlyde or some other high quality Silicon based brake caliper grease, not the generic permatex type stuff.

Change the fluid.

Check the calipers during tire swapping (all season to snows) or during rotations - in the rust belt this is wise.

Never worry about calipers again.

Edited by grossgary
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Well, I can't fully disagree with you. I was perplexed myself with this one, as I realize even in the rust belt calipers last for years and years, at least five or more mostly.

The car belongs to a relative, so, maybe it was brought to another shop, and they simply took the nicely rebuilt calipers

 out, kept them for themselves, and replaced them with old rusted ones. Who knows for sure. But all of the hardware was replaced as well. All of the boots for the pins, new pins,

 and, top quality extra high temperature guide pin lube as well. Just one of those things I guess. Also it could have been maybe the worn rear wheel bearings caused the rotor

 to wobble (there were new rotors installed as well with the calipers) and somehow jammed the piston in the caliper, compromised the seal, and water got in, along with road salt,

 and froze it shut with rust. Still leaves me in shock though how short the lifespan was of those calipers. Took the rotors with them too.

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  • 4 weeks later...

Digging up an old thread here, but there are a couple of points I think I can add to.


First, in rust-prone areas some people have upgraded to stainless caliper pistons (where available). I know for sure the front 4 pot calipers (2005-06 WRX in the US) are available, I don't know on the 1 pot Forester rears).


The second point is the WRX rear 2-pots (non-Brembo / STI) are alloy, not cast iron - the body, not the piston! So that may resolve your corrosion issue.


I have been personally rebuilding a couple sets of the 4-pot front calipers that came out of the UK, which would be a similar climate to you (I am in Australia, we don't see your weather here :)) - and one set was terribly rusted. I had to pull them down to individual parts, 2 of the pistons were stuck in place and had already been vice-gripped by the previous owner so were damaged. I ended up twisting them out using vice-grips and a breaker bar. The rest popped out using compressed air but were rusted through. I made one good set out of 2 sets of calipers and replaced the other set with new pistons. I then had the calipers chemically stripped, then rust-treated, then wire-brushed all over. The cast metal will rust overnight if left outside, I am going to have them painted in 2-pak to stop the rust from re-starting. I have used rattle-pack caliper paint in the past and it works but it stains easily while 2-pak is there for good.


The rear 2-pots on the other hand (which were sourced locally) need nothing at all - they are factory anodized black and still look as good as new. If you go down this path you will need to change your backing plate or use kartboy adaptors with your existing backing plate (I did the former, it is not difficult to change the backing plate).

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