Brake Caliper Piston Wear

[crazyramen]

I'm getting around to the brakes in my 79 GS750 and honestly they didn't seem as bad as I expected. Not much rust on em even though the boots were mangled. The pads were down to the last mm and the rotor is well below spec. That's all stuff that's pretty straight forward for me to diagnose.


ANYWAY my question comes in on the pistons. They're within spec dimensionally but they've got some wear on them. Both have enough scoring that I can catch a nail on it and the inner one has some small pitting on it. I think it's enough that I should replace them outright, but I was hoping to get a second opinion from you knowledgeable folks before I spend the cash!

I can only upload one photo at a time here so here's some more ok imgur: https://imgur.com/a/Wt76ADI

[Nessism]

If the rest of the pistons look like the photos, I'd reuse them. Polish with 600 or 1000 grit sandpaper before reinstall.

[crazyramen]

Thanks for the tip! It's hard to find pictures online of "acceptable" wear - all my searches only brought up comparisons between brand new and very badly work.

[chuck hahn]

Looks like maybe sand or other grit got past the dust boots. 'Try' the polishing first and see if they eventually leak or not. But at a minimum id replace the piston seals. And be sure to clean the seal groves as spotless as you can, i use the dremel and a wire wheel. ALSO when you install seals reach into the well and feel the back side of the seal. You should feel a little ridge. Seals have a tiny taper and the flush side faces out to facilitate sliding the piston in. Lube piston with some brake fluid then install.

[crazyramen]

Looks like maybe sand or other grit got past the dust boots. 'Try' the polishing first and see if they eventually leak or not. But at a minimum id replace the piston seals. And be sure to clean the seal groves as spotless as you can, i use the dremel and a wire wheel. ALSO when you install seals reach into the well and feel the back side of the seal. You should feel a little ridge. Seals have a tiny taper and the flush side faces out to facilitate sliding the piston in. Lube piston with some brake fluid then install.

Definitely gonna replace the seals. This bike has been sitting for quite a while and the boots are totally blown out.

Looking more into the seal groves on the caliper, there's quite a bit of hardened crud in there. Sorta looks like somebody tried to seal it up with something that reacted poorly to the brake fluid. It's almost the same consistency of whatever they used to seal up the air box that has hardened to a rock over time.

I'll have a go at it with the Dremel wire wheel, polish up the pistons nicely, and replace all the seals. We'll see where that leaves me!

[Nessism]

Be sure to replace the brake lines. They are sure to be filled with scale on the inside...

P1010652 by nessism, on Flickr

[chuck hahn]

Brake fluid draws moisture to itself. Thats why it needs changed every two years. And that hard chalky scale is a result of moisture getting in and causing oxidation. The oxidation actually "chokes" the seals tighter to the pistons over time causing them to stick and drag. I also put a wipe of brake fluid on seals to help them slide into the grooves and seat well before putting the pistons in.

[crazyramen]

Be sure to replace the brake lines. They are sure to be filled with scale on the inside...

P1010652 by nessism, on Flickr

That's pretty eye opening. I'll have to find some replacements/learn how to make em.

Brake fluid draws moisture to itself. Thats why it needs changed every two years. And that hard chalky scale is a result of moisture getting in and causing oxidation. The oxidation actually "chokes" the seals tighter to the pistons over time causing them to stick and drag. I also put a wipe of brake fluid on seals to help them slide into the grooves and seat well before putting the pistons in.

That totally makes sense. I always wondered where the gunk in brake lines came from and I knew it was hygroscopic, but I didn't realize that's what caused it to gunk up.

[Nessism]

One option for new brake lines...

https://www.thegsresources.com/_forum/forum/technical-forums/popular-technical-info/84535-earl-s-brake-line-thread

[chuck hahn]

Look for a local shop that makes hydraulic lines. If they dont do brake lines they may know someone that does. Im lucky enough to have a place 10 miles away. Full set for the dual front discs was 136 dollars last set i bought.

[1978GS750E]

I've been using Apex brakes for a while now with excellent results. Contact Raymond Chan @ raymond@apexbrakes.com

My last set for the '79 GS850G was $138 for the complete set of 4 lines.

[crazyramen]

One option for new brake lines...

https://www.thegsresources.com/_foru...ke-line-thread

Woah thanks for that awesome write up I'm gonna be reading the rest of those "popular technical info" posts over the next few days.
I might go that route - seems like a piece of cake compared to the bending/flaring/routing nightmare that I've gone through for automotive brake lines. It's gonna depend on cost. My initial check is sitting at about $145 for 4 fittings, 10' of hose, a few extra ferrules, and some crush washers. Seems like I can get some cheaper (and I'm sure lower quality/Chinese) parts from Z1 or ebay - so I'd probably go with those.

Look for a local shop that makes hydraulic lines. If they dont do brake lines they may know someone that does. Im lucky enough to have a place 10 miles away. Full set for the dual front discs was 136 dollars last set i bought.

Now that you mention it Chuck, I think the NAPA across the street from my house does hydraulic lines for heavy equipment, so I'll see if they'll do some brake lines for me. I assume they'll be the cheaper rubber lines, but that's gonna be ok with me if the cost is significantly lower.

[chuck hahn]

Yes theyll probably be DOT rubber lines. Ive tried stainless and TO ME theres no super noticable difference. OEM rubbers as good in my opinion for normal use.
Ill bet folks with of classic cars have sources for lines too. Ask around and youll find a place,

[Nessism]

The nice thing about building your own lines is you can customize the installation. Pick whatever hose end angles and length you desire. For example, some people like to run one line to one of the front calipers, then a second line jumps from left to right, to the other caliper. Something like that.

Last time I built lines, Earl's was the cheapest option. You don't need the adjustable hose ends, just clock them carefully, which saves money too.

[crazyramen]

Ok the pistons look quite a bit better after cleaning up. The pitting on the inner one is still there but I'll run them for now and see how they do.

I talked to a couple of spots in town and it sounds like it'll cost me as much to get the rubber crimped lines locally as it will for me to build PTFE/braided steel lines - so I'll just diy it.