Rear caliper drag (rebuilt, new ss piston, seals etc): axle alignment?

[DiverRay]

You may be able to fine a hone tool at the auto parts store. A lot of the "imports" use the smaller bore and someone may have the hone for it.

[DEEBS11]

It can't be silica. It has to be aluminum oxide or you will ruin the bore.

https://www.ebay.com/itm/234292184532

[gtem]

Ah, that's the exact one! No idea why I couldn't find that myself.

So last night I went through both the caliper and M/C again, recleaned the caliper seal groove even more, polished the caliper bore a bit with 1000 grit, checked the spooge hole on the M/C, bolted it all back together and rebled using all the tips/techniques I've now learned (one I've had good luck with is pumping the piston out a good ways then driving the piston back into the bore with a screw type caliper tool, you can see the bubbles come up into the reservoir). A test ride had the rear rear rotor merely warm, and the rotor warpage was much less of an issue. Pedal was still a bit mushy at this point, but after the test ride I just bled it a bit more and now the pedal feel is nice and actually the rear wheel is freer yet. I can spin it a full rotation by hand (grab it and get it spinning then let it go), rolling it around the garage it feels much better. One thing I have been noticing is that a bit of brake fluid will end up in the M/C dust seal, and that when pumping the M/C you can tell a bit of "suction" forms on the dust seal. So I'm not out of the woods yet, I do need to properly hone that M/C bore and hope that finally makes a proper reliable seal.

[gtem]

A further update: after sitting in the garage a few days, the caliper has gotten tighter again. I'm fairly certain it's down to that misbehaving (leaking) master cylinder. I've got the correct style ball hone on order, fingers crossed I can clean up the bore on one of my two master cylinders. Buying them used off ebay is a bit of a fool's errand I'm learning, they seem to *all* have some pitting in the bores.

I also worry about about whether the new seals on the rebuild kits are getting damaged with all this being disassembled then reassembled, or dragging on substandard surfaces... is it at all possible to buy just the two rubber seals rather than $30 rebuild kits with the metal piston, spring, etc?

[gtem]

One more update for anyone interested in this saga (or finds this thread years from now dealing with the same problem)

I ended up buying the 1/2" Al-oxide ball hone as suggested, I think it worked great. Reassembled and bled everything. Have not seen any leakage from the master cylinder. Have been riding the bike to work. I'll say there's still a *bit* more drag than I'd say is ideal, the rear rotor still gets hotter than the fronts, but as I've been riding it it's been getting better it seems. The wheel spins fairly well (almost a full rotation when I give it a good spin by hand). I think the remaining culprits are a) the new piston seal is just a hair thicker than an OE, my local vintage bike friend (from messing with GS Suzukis) said he had this issue with new aftermarket seals before. His solution was to take some emery cloth to the ID of the new seal. Additionally, I think my rear rotor *is* a hair warped from when the rotor got really hot when this whole issue initially started with the sticking caliper. I think I'm just going to let it ride, and the seal will hopefully continue to wear itself in.

[DEEBS11]

Ya gtem, the fact that your buddy suggested abrading a rubber seal shows that it's time for him to stop wrenching. He is now an official hack. Aside from that, if you were to remove a slight amount of material, it would be from the aluminum seat where the seal rests. That is the area that builds up aluminum oxide and makes the seal grab and squeeze the piston. Just like I pictured here:

[DEEBS11]

Another thing to mention is to never push fluid pressure towards the master cylinder in trying to bleed air out of the system. The check valve will try to block it and you will build up pressure behind the piston that won't release.

Always draw fluid toward the caliper.

[LAB3]

Another thing to mention is to never push fluid pressure towards the master cylinder in trying to bleed air out of the system. The check valve will try to block it and you will build up pressure behind the piston that won't release.

Always draw fluid toward the caliper.

I've owned three Yamahas, a Goldwing and a Suzuki GS since I learned about reverse bleeding and have never encountered that issue.

[DEEBS11]

It happened to be on another brand. I had to crack the bleeder to release the pressure and bleed it towards the caliper and then it was ok.

[gtem]

Well I pulled things back apart to really polish up that seal groove, put it back together, had the usual fight bleeding the thing (still has an air bubble judging by my mushy pedal) but hey, the rotor is spinning as freely as it ever has. We'll see if it stays that way once I get all the air out.

[DEEBS11]

This is the check valve found in every master cylinder. It insures the fluid only flows in one direction. Out to the caliper. Fluid can be forced past it and I have done it in the past but it can cause problems. Afterwards you must bleed towards the caliper to remove any pressure that has built up behind the caliper piston.

As you can see, the holes are very tiny and the check valve washer is very thin. If you are cleaning, this is always gunked up. It is often overlooked.

[DEEBS11]

One-way check valve.

[gtem]

Well I pulled things back apart to really polish up that seal groove, put it back together, had the usual fight bleeding the thing (still has an air bubble judging by my mushy pedal) but hey, the rotor is spinning as freely as it ever has. We'll see if it stays that way once I get all the air out.

Bled the system a bit more, nice feeling pedal, drag on the rotor is +/- what it was before this latest attempt to polish the groove. I'm gonna call this good enough for my ride out to AMA Vintage Days next week.

[bikerphil]

An old trick to get the last bit of air out of the brake system is to place a wood block in between the pedal and the foot peg, keeping the pedal depressed. Leave it this way overnight. Any micro air bubbles should escape back into the air chamber in the M/C reservoir. This also works with the front brake by tying the lever back with a bungee cord and leaving overnight.