Sprint RS rear caliper binding

[slowside]

So I rebuilt front and rear calipers with new seals and pistons about 200 miles ago.

I’ve noticed an audible drag on the rear brake when wheeling the bike around and on stopping after a ride the rear disc is hot. Not molten metal, melts ice creams in a 5 mile radius hot, but don’t keep your hand on it too long.

I plan to give it another bleed in case there is an air bubble preventing pressure releasing into the master cylinder.

Any other culprits I should be on the lookout for?

I should add that whilst the front calipers cleaned up nicely, the rear took hours of work to remove the grime from the piston and seal seats and I never got it spotlessly clean.

[NJD]

Give them a clean. Out of all the work I do myself cleaning the calipers is the job I do the most, and more when it rains or during winter.

If you can replace pistons and seals and flush the system without issue then I'd be surprised if you said you haven't pumped the pistons out (partly, and not all the way), sprayed them with brake cleaner and then regreased all the sliding parts and back of pads before making this post?

Looking at the face of the pads will tell you if they've been rubbing heavily for a long time, but that's just one of a few signs.

The rear is probably always going to suffer more just because that's where all the road crud is kicked up.
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The do it all, T̶h̶e̶ ̶b̶r̶o̶k̶e̶n̶ ̶o̶n̶e̶,̶ ̶T̶h̶e̶ ̶b̶i̶g̶ ̶l̶u̶m̶p̶,̶ ̶C̶h̶o̶n̶g̶ ̶N̶o̶o̶d̶l̶e̶

[steve the grease]

Very often there is a bit of white aluminium oxide in the groove where the rubber goes, this causes the rubber to be a bit proud in places, this squeezes the piston tighter than usual , making the piston not go back quite so easily / as far. Strip and clean again, with particular attention to the grooves.
A local wheel refurbishers would probably bead blast the bore in a minute of two for a fiver or a tenner.
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All the above is my personal opinion, you can see my lips move, but I'm talking out of my arse.
I've been riding, and fixing , bikes for 50 years, in that time the more I learn, the less I am absolutely sure of.....

[Robby]

Either corrosion behind the seal in the caliper, or something blocking the return hole in the master cylinder.

Also make sure your pads can move freely, and the pad pins aren't bent, notched or rusty.

[stevo123]

Also don;t overlook the basics - is the brake pedal allowing some clearance to the master cylinder when not pressed with your foot? Must be a small bit of "slack" between the resting position of the lever and the master cylinder else pressure can build leading to binding.

Not as likely as corrosion or dirty piston/slider though but keep it in mind mate.

[slowside]

Thanks all, yes must admit that at the point I posted this thread I've not done anything beyond parking it up in the garage - but thought I canvas some collective wisdom before I start on the basics of checking it over this weekend.

The rear caliper itself is still visually clean and during the rebuild I replaced absolutely everything bar the caliper block itself, so in addition to pistons and seals it has new slid pin, rubber boots, bleed nipples etc.

That said I'll pull it off and give it another clean up on Sat and give the brake fluid another flush through.

Hopefully I can avoid having to take apart the caliper again. Was a messy ball-ache of a job.

[steve the grease]

[NJD]

[slowside]

Much appreciated, just the kind of info I was after.

When I acquired this bike it was in quite a neglected state, I've done all the work on it myself to get it roadworthy, but most of these jobs are a first for me (caliper rebuild included) so I appreciate the insight from those who have been there and got the t-shirt.

My old YBR was a doddle in comparison, was barely any different to fixing a bicycle.

[Robby]

My favourite tools for cleaning calipers are a drill and a pick.

The drill is for sliding pins. Clamp them like a drill bit, spin, and hold some emery paper over the pin. Gets them clean and keeps them round. A bit like a really cheap lathe.
Ideally they should be replaced - they are wear items. The problem is they cost a lot for what they are and invariably only get sold from places that do really slow postage.

The pick is for remove the seals in the caliper, and then cleaning out the white crud from behind them. You don't need to be too gentle here - the white crud is very soft, the caliper body is fairly hard, and if you press to hard you'll bend/break the pick before hurting the caliper.

Get these bits right and the rest is less important. It's very easy to get anal about using just the right amount of grease, when it makes very little difference in reality. Too much and you'll make more of a mess when the dirt sticks to it. Too little and things may corrode a bit faster - but it all corrodes worryingly fast anyway. I also wouldn't worry about having the exact right kind of grease, for instance I have never used red rubber grease. Getting things clean and smooth is more important.

Finally, it is important to note that bike brakes, particularly rear brakes, tend to be a fairly shit design. The brake lines run almost horizontal, which means bleeding them is very difficult and best done with the caliper off the bike, either sitting on the floor or elevated so the air can go somewhere (either up to the master cylinder, or up to the bleed nipple). They get dirty and corroded, but don't have rubber boots over important bits. They also get used so little that rear pads tend to last years, which means a good chunk of the piston is sitting there exposed to the elements and gently rusting for years at a time. Then to make it more fun, you have to dismantle half the back end to remove/replace them. Shit job.

Because of this, it's worth doing all of the jobs at once. Pad change, strip and clean, fluid change, bleed.

[Kickstart]

In addition to the above, make sure you haven’t over filled the fluid reservoir.

A a few bikes have seals that are tapered and only go in one way. Not common, but can screw things up if the wrong way round

All the best

Katy
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[slowside]

Quick update:

I pushed out the pistons a bit and gave everything a clean (they were pretty clean anyway) and replaced the pad retainer pins with new ones, additionally I took a few ml out of the reservoir as it was on the full line.

Visually I can see the pistons slack off when pressure is released from the brake pedal.

Disc only feels warm to the touch after a ride now, so if that heat isn't from normal use any drag is pretty minimal.

Cheers all for the advice.
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Triumph Sprint RS 955i - Sold
Triumph Street Triple R 675

[MCN]

[slowside]