Caliper rebuild inquiry

[Momaru]

Hi all. Been a while since I've been active, but I'm still trolling about Finally had another maintenance issue/inquiry I couldn't solve with search or my FSM.

Long story short, one of the pistons on my front brake caliper siezed a little this past summer. I wound up disassembling the caliper, hitting the piston with some steel wool to clear the corrosion, cleaning up everything best I could and puttin it back together. After I figured out I needed to back-fill the caliper with fluid (couldn't generate enough vacuum to just bleed the fluid in), it seemed ok. It works, but been rather squishy ever since. I know, that suggests I've still got air in there somewhere.

I've bled at least half a liter of new fluid through the line 5 times or more. I continue to get significant amounts of bubbles once I pull a vacuum, but just using the pressure from the master cylinder, no bubbles. I'm thinking I've blown a seal, either because of my 'maintenance' or by using a hand-operated vacuum pump. There is no fluid on the piston itself after a ride, so I find myself rather confused as that seems to be the diagnostic for a broken seal.

I swapped to stainless lines this past weekend. Swap on the rear went off without a hitch, including replacing my no-longer-functional brake light switch banjo with a new one and shortening my fluid reservoir tube to stop it rubbing the tire. Front seemed ok, but braking power is still very lacking. I pulled the caliper off the mount and turned it every which way, tapping to get any air bubbles out, but the problem persisted: once I start vacuum, I get about 1/3 of the tube volume continuously filled with small bubbles (and yes I was keeping the reservoir filled). Used fresh out of the bottle DOT 4 from Valvoline, just in case the stuff I put in 4 months ago had soured.

I'd rather fix the caliper I've got than buy a new-used one online. At least I know where this one's been and who did what to it when.

Thoughts? Anything else I should check? I'm frankly a little confused.

[Racer x]

When I have a really stupid hard break to bleed. I pull the caliper and master. And get the caliper above the master cylinder. Then bleed it like that with a piece of wood between the pads . Usually rear brakes. Just get the bleeder at the top and the air has to go to it.

[alex.s]

+1 take the caliper off and hold it above the master with a sealed tube on the bleeder leading up to a hanging catch bottle and pump it and shake it untill no more air bubbles come out and the fluid is up higher than the caliper... then close the bleeder with the fluid still in the tube

[Motofool]

Originally Posted by Momaru

......I've bled at least half a liter of new fluid through the line 5 times or more. I continue to get significant amounts of bubbles once I pull a vacuum, but just using the pressure from the master cylinder, no bubbles......

You may have some leak only downstream the bleeder, from which you vacuum machine is sucking those bubbles.

If the pressure does not show bubbles and the brake is still weak, you may have some air trapped inside the system (that is not pulled by the vacuum machine).

Do as the gentlemen above have suggested.
Think of air inside like balloons trapped inside a house but willing to fly high if nothing stops them.

If you blew a seal, you should had seen abundant fluid leaking out from the caliper.
You repair should be OK.

[Momaru]

Thanks guys. I will definitely give that a go. Thought about it the other day but it was getting dark and the more I thought about it, the more it worried me because I spent at least two hours bleeding that line and continued to get bubbles.

I'll give the "bleed it with the caliper high" method a go and let ya'll know.

Motofool, not sure I understand your first sentence, but I get the message.

[Racer x]

When using a vacuum bleeder air is often pulled around the threads of the bleeder. To prevent this. Pull the bleeder out and put a small amount of grease on the threads.

[bruce71198]

I was just going to mention that Racerx, I use the vacuum bleeder on a daily basis and air past the bleeder screw threads is comon. This doesnt mean there is air in the brakes as this air is pulled in after the bleeder seat. Other areas for air to hide are at the banjo bolt on the master, usually when I'm pretty sure the system is bleed completely I will zip tie the brake lever to the handle bar and let it sit this way for hours. What this does is let the air travel to the highest part of the system under pressure, then when you cut the zip tie the master pulls a good amount of fluid and that residule air back into the resevour. This proceedure has proven fail safe for the final step in my brake service.
Being that you get no bubbles when pumping the master makes me think something else is causing your spongy brake. To much piston travel caused by other sticking parts in the brake system. Particularly the slide pins in the calipers and the various spring plates on the caliper and pads installed wrong.
I find that any part that doesnt slide smoother than deer guts on a door knob is not acceptable !!

[Motofool]

Originally Posted by Momaru

.........Motofool, not sure I understand your first sentence, but I get the message.

Sorry, my English is very poor.

What the two last posts explain is what I meant.

[Momaru]

Well, after being sick, travelling, crappy weather and having to work two consecutive weekends, finally got to work on the brakes again. Long story short, you were ALL right. I was messin' up in a bunch of ways. That'll teach me to just assume it's as easy as it looks

Turns out there were a number of issues.

Originally Posted by Racer x

When using a vacuum bleeder air is often pulled around the threads of the bleeder. To prevent this. Pull the bleeder out and put a small amount of grease on the threads.

Originally Posted by bruce71198

Particularly the slide pins in the calipers and the various spring plates on the caliper and pads installed wrong.

Originally Posted by Motofool

You may have some leak only downstream [from] the bleeder, from which you vacuum machine is sucking those bubbles.

Motofool, your English is just fine; far more comprehensible than some native speakers I know. I wish I was sufficiently fluent in another language to be able to provide technical help. Once the others explained it in terms that my befuddles brain was able to grasp, your statement made perfect sense too. Also, you (and Racer X, and once again Bruce71198) were all correct; I was pulling lots of air past the bleeder threads. Once I put a dab of grease on the threads, it stopped.
Fixing the seal on the reservoir for my cheap little vacuum bleeder helped a ton too.

The "squish" I had been feeling wasn't exactly from the brake fluid/air. It was (at least in part) the fact that I didn't properly re-set the slide pins (thanks @bruce71198) when I did the work earlier this year. I'd trapped a bunch of grease (I put in far too much) & air down at the bottom of the metal sleeve. It would compress a little, then stop, still allowing me to use the brakes, but just a lot more spongy than they should have been.

I did try the "bleed with caliper high" method and that did speed things along a good bit, esp for the initial re-filling, so thanks gentlemen. However, how in the heck do you get everything re-assembled on the bike without having to re-bleed? I've got the brake line threaded between the forks (beside the throttle cables for a little ways), per OEM routing and I could NOT get my master cylinder through there for love nor money. Wound up just separating the line from the MC, routing it, reassembling and bleeding it a bit more. Not a big deal, just a bit of head scratching.

Since I had the time over break, I just ordered new rubber bits since it's past time on my "recommended factory service" schedule anyway. In the process of the "official rebuild" I discovered that if you bottom out the pistons when you re-insert them, they can be a little jammed inside the caliper by getting just a little off-axis at the end of their travel. I had put everything back together and just couldn't get things to bleed or get any compression from the pistons. So, I took everything apart. Again. Used my compressor to un-stick the pistons, re-seated them at a normal depth and bled again, everything worked smoothly.

After another day of fiddling with this (and re-bleeding the brakes for the ~8th time this year), and several face-palm moments, it's finally done right and feels the way it should. Have a zip tie on the brake lever to get that last little bit of air out overnight.

Thanks for your help everyone. I sit here a much wiser and happier rider because of your efforts.

[Domagoj]

It's a bit late now, but maybe worth mentioning. I had some problems with weird front brake feeling.
It turned out to be those 2 holes in the brake fluid reservoir which supply and return to/from the cylinder. I had some dirt in those, not sure how it got there though. Not a chunk of material, stuff that stuck to the sides and decreased the hole cross section. In any case, after cleaning those holes, my front brake finally became normal again. Numerous caliper cleanings, flushing and bleeding before that did not help much.

I'm glad you fixed yours.

[Momaru]

As an additional note... unless you want a large crack in your gauge cover lenses, DO NOT get brake fluid on them. Second time I've had to replace that in a year. FWIW, first time was from lowsiding at the track, which is what kicked off my "rebuild the brakes" adventure.

Yeah, just don't do it and make bloody sure you got every little tiny drip off the brake line

[Motofool]

All very good tips, Paul; thanks.

[Whiskey]

Originally Posted by Momaru

Since I had the time over break, I just ordered new rubber bits since it's past time on my "recommended factory service" schedule anyway. In the process of the "official rebuild" I discovered that if you bottom out the pistons when you re-insert them, they can be a little jammed inside the caliper by getting just a little off-axis at the end of their travel. I had put everything back together and just couldn't get things to bleed or get any compression from the pistons. So, I took everything apart. Again. Used my compressor to un-stick the pistons, re-seated them at a normal depth and bled again, everything worked smoothly.

Probably a bit late for you but to anyone else trying to fit pistons back into a caliper:
Place a piece of wood onto the piston faces & use a G-clamp to wind them back in slowly & evenly. The wood keeps pressure on both pistons & prevents the clamp from damaging them.