front caliper piston will not go in easily

[Partscaster62]

Hi - bought a 79 xs1100SF - my first bike! I am rebuilding the front brake calipers. Have already done the MC; I cannot seem to get the piston to go back in easily. I have been reading through some posts in the forum on this. I used a Dremel wire wheel on the caliper groove and a pick. It looks very clean - piston is clean and not pitted. I used clean brake fluid and even tried some silicon lube spray. I made the mistake of pressing it in hard once and had to use the zirc gun to get it back out. The rebuild kit is K&L. Any suggestions? Thanks!

[Diskman01]

If you loosen the bleeder does it go back in easily? If so, then the "return" hole in the master cylinder is plugged.

[Partscaster62]

Thanks for replying - The bleeder valve is off right now - maybe still some grease still in the caliper from having to "zirc" the piston out? I will check it out!

[Partscaster62]

still not working. both holes are cleaned out - the caliper is not hooked up to the banjo bolt and does not have the bleeder valve in at all. I've even tried to flip the piston seal upside down. It's still real tight. The piston slides in very smoothly with no seal.

[jetmechmarty]

I suppose it's possible that there was a quality control issue with your kit. It didn't come with that red brake lube?

[Partscaster62]

No. I also tried the ring seal from the other kit (for other caliper) and it did not work either.

[bikerphil]

Compare the old ring with the new one and see if they match.

[Partscaster62]

Appears to be exact match. The old seal does not even appear even worn. But they were all cruddy and locked up when I started the rebuild.

[Partscaster62]

Should I try to reuse the old seal? My assumption at this point is a big NO.

[Bonz]

I helped Sebastian_99 clean and service the front calipers on his 79SF a month ago, and it was a biotch getting the pistons back in there.

No harm in using the old seals if they are in good condition, but if you have new ones no harm using them either.

We did not replace any parts, just cleaned up good, cleaned out the o-ring groove in the caliper bore, cleaned the buildup off the o-ring itself,etc, etc, etc.

Used brake fluid to lube everything, lube the heck out of the o-ring, bottom half of the piston and lube the caliper bore really good. I ended up using a set of channel lock pliers carefully padded with a shop rag to get the pistons to seat fully. We could get them in about 1/3 of the way by hand, fwiw.

It is simply the tight tolerances, nothing more in my opinion. Some are easier to seat than others, and you have one of the "others"!

[Partscaster62]

Well, I can get it in, even just but I was thinking "easily" was the key term. One post I read said they should slide in like a "dream". I can push them in all they way by hand. Won't they still be "grabby" if they don't go in easily?

[MPittma100]

Define

Well, I can get it in, even just but I was thinking "easily" was the key term. One post I read said they should slide in like a "dream". I can push them in all they way by hand. Won't they still be "grabby" if they don't go in easily?

Might need to define "easily". There is going to be some friction between the piston and the seal ring. I always use a caliper piston tool to seat the piston.

You have verified that the o-ring grove and bore are clean and the piston slides freely w/o the o-ring. I don't think that there is a problem. Put it together, bleed the system. See what happens.

MP

[Partscaster62]

Thanks guys - I move forward with the last suggestion and let you know how it goes!

[Bonz]

That tight seal that is so hard to push against with your hand strength alone, is your friend when it comes to hydraulic pressue pushing the piston outward to the pads. Smoooooth.... unless something else is amiss...

Where brakes get grabby is a gunked up groove under the caliper bore o-ring. The o-ring is ever so slightly unseated inward at one or more points and that causes the piston to be grabby when it tries to retract.

[Courtney]

Here's my experience with the ORIGINAL seals after cleaning everything surgically clean! No matter how much I tried, the piston was NOT cooperating. I tried Syl-Glyde brake grease, it was the worst of anything as a lube; too thick. Tried some brake grease my buddy gave me, it was better, but STILL not doing anything. Finally, I got out my spray can of Silicone Spray and hosed the caliper inside, the seal, and the piston with it. Lined up the piston carefully and was able to push it in with plenty of thumb pressure on each side. It wasn't an easy slide in, but it was able to go in.
Try silicone spray liberally and push hard with your thumbs. Worked for me. Remember all I did was re-use the old seals and it was STILL a bitch!