Caliper piston boot

[xwhatsit]

Fargin' shissen ferk blargh foo cack!

Checked my brake pads today; looking a bit sad, make a note to order some new ones. I realise I've never done pads myself; how to do? Ask Mr Haynes. I decide; why not do a dry run now to see what it's like, then I'll put the new pads in when they arrive. Yes I was feeling sleepy and stupid.

Just before I pull the pins out that hold the pads in, I'm supposed to push the piston back into the caliper, so I can get the pads back in later. I use a big screwdriver to lever it back in.

I do that.

Take pins out (one was stuck in there, hammer time!), pads out, give them a bit of a rub with some sandpaper and stick 'em back in.

Put everything back together, notice something. The rubber piston boot has a tear in it

What the fark!!!

What does this thing do? Is it just to protect the piston/bore? How easy to replace? When I pumped the brakes back up, no fluid came squirting out (if it did, that would mean the seals are farked, no?).

Way to ruin my day

[vifferman]

Yeah, the seals are there to protect the bore of the pistons AND to provide a bit of assistance to the pistons retracting when you let off the brake lever (or so I was told on a biker forum).
They're not expensive, but may have to be ordered (like when I bought some for my VTR, but then Honda parts are NEVER in stock )

[imdying]

They do bugger all, you can ride without it. Just order a kit in, and then recon the caliper when it arrives.

[xwhatsit]

I'm guessing that when I recondition the caliper, I'll find the piston is gouged to hell (it'll be the original from the 1980 running gear on this bike). Know anywhere where you can buy cheap caliper pistons? I've seen them on UK sites, generic ones categorised by 27mm, 28mm 28.5mm etc, for 15 pounds. Anywhere cheap locally? If I buy OEM I'll be out a hundred or two I'd expect.

Cheers guys. I knew you'd turn up Imdying -- see, this is why I hate hydraulic disc brakes

[Eurodave]

I had need of a couple of Honda caliper pistons a while ago, so just took one of the old ones to a mate & he machined a pair from stainless, sweet as & a fraction of the OEM price

[imdying]

Typically pistons are priced around the $2 per mm of diameter to resleeve in stainless steel. It's a common practice, and I've never seen a sleeve leak or give any trouble, and I've seen literally thousands of them done.

As Eurodave says, you can have them machined from stainless and they'll work fine, it you know someone in the trade. You could also machine them from brass at a pinch, I've seen hundreds of those working fine in boat trailers, where they were quite popular for their anti corrosive properties... until phenolic ones became the norm.

Closest to you for caliper piston resleeving would be Apex Engineering in Hamilton, they do fine work imho.

[xwhatsit]

I'm not totally in with having it re-sleeved (single pot, yay, cheaper ), as it would mean having to take the bike off the road for a week or so, which I can't really afford (catching the bus to get me to work and uni would run $100 or so a week). Now that I know it's not about to kill me by having this piston boot torn (meaning I won't have to catch the bus anyway), I'm definitely keener on buying a new piston so I can rebuild the thing in a day and be up and running straight away.

Just got off the phone with Malcolm from Econohonda, a new boot is $23 and hydraulic seal $45. He says he can get a piston for $75-odd aftermarket, much more OEM, which isn't as bad as I thought. Will see what I can find in the mean time.

Now to find out what the hell dimensions the cylinder is, hehe.

[imdying]

That is rape. I accept the $23 for the boot, you can't get them anywhere else, but the seal should be around $10, or $4 for an aftermarket one. Piston price is fair, I suspect (would hope) that at $45 you must be getting slide rubbers and boots too, which would be quite nice to have. I wouldn't have though Apex would take longer than a day to do your piston. Your other alternative is to ring someone like BNT, have them track down a piston of the same diameter as yours, and then turn it down on a lathe to the correct length. Of course, you may be better off ringing a brake specialists, as they're often more helpful than franchise places, and they'd probably have a lathe they can turn it down on.

[xwhatsit]

I love you, Imdying. You're smart and pretty

From his phrasing, I suspect that was only the hydraulic seal. He said something about `it's a stupid price', so it seems Malcolm agrees with us there.

I'd like to replace all the rubber bits, as I just know they'll turn into dust as soon as I take it apart.

Does anybody have any recommendations for brake specialists up this way? If not I'll do the usual Yellow Pages trawl. I noticed, while searching, in a very old post here you mentioned that car rear brakes are often a similar size and thus you can use normal automotive brake people.

[imdying]

Typically the piston seal in an automotive hydraulic disc brake caliper is:

- Made from a glycol based rubber (compatible with DOT 3,4 or 5.1 brake fluid, but will die in short order if anything petroleum based gets near it, say CRC, petrol, trans fluid etc)
- Will be of a square section (typically only Mk1 Trojan trailer calipers, and Willwood car racing calipers, use round section orings). This is because the square section design utilises the shearing effect of the piston dragging it out, to pull the piston back in when the pressure is off, as the seal wants to make itself square again. This is one of the main reasons why pistons 'drag' when the seals are old. The other main reason is dirty, which interferes with the piston retracting into the body, and interferes with the self righting nature of the square section seal.
- Caliper seals are always measured on the ID, or specifically, the OD of the piston.
- Caliper seals are almost always 1/8" section square. There are some old balls like old Chryslers that use 5/32" section, but I've never seen anything else on bikes (well, except for some smaller section ones on some small calipers)


Seals as described above can be obtained from most brake specialists (http://nzbcsa.org.nz/ for locations), for example:
- A 1-1/2" OD piston would utilise a P4729 seal (PBR numbers, as commonly used in the industry)
- Sizes generally go in 1-1/6" steps, better off slightly too large than slightly too small, the bore will compress a slightly too large seal onto the piston, so it won't leak.
- When checking the seal for size, you'll notice that it's not a snug fit to the OD of the piston, but it will lightly drag up and down it (by hand)

Typically on a 4 or 6 pistons (or 2 pot sliding) calipers, rear caliper seals are often a close match for size (1-3/16" or 1-1/4").

I have seen situations where the OEM ones were a slightly smaller than 1/8" section, and using car ones causes the pistons to bind. That isn't too common though, and probably won't be any problem at all with your larger single piston caliper.