Topic: '78 750K caliper seal part #

[BlindJoe]

Could someone tell me the part # for this please, I've ordered two diff. seals from part #'s I've found on here and neither seem to fit. One's too big other one seems too small. This is one of the # I've tried, 45108-300-003, I can't find the other atm.

Thanks

[mick7504]

This is the seal listed for the later caliper with no ribbing on the outside of the housing.

http://www.cmsnl.com/products/45215-533-004-sealdust_45215533043/

Mick

[BlindJoe]

      Thanks for the reply, I'm pretty sure that's the one I've got in there now. I cannot get the brake to harden up for the life of me! I've cleaned the caliper and put the new seal and brake pads on, installed new ss brake lines, and have a rebuilt Nissin 1/2" master cylinder. I've tried everything to bleed them, and I'm almost positive I've gotten all the air out, the lever gives some resistance, but I can still pull it all the way to the grip! I've spent probably 10 hrs trying to get this setup to work. I don't know what else I could do. Any thoughts would be greatly appreciated, thanks.

[mick7504]

      Thanks for the reply, I'm pretty sure that's the one I've got in there now. I cannot get the brake to harden up for the life of me! I've cleaned the caliper and put the new seal and brake pads on, installed new ss brake lines, and have a rebuilt Nissin 1/2" master cylinder. I've tried everything to bleed them, and I'm almost positive I've gotten all the air out, the lever gives some resistance, but I can still pull it all the way to the grip! I've spent probably 10 hrs trying to get this setup to work. I don't know what else I could do. Any thoughts would be greatly appreciated, thanks.

The 1/2" bore master cylinder could be your problem mate.
The stock bore size is 5/8" and you quite possibly aren't getting enough fluid displacement from the smaller master cylinder.
I feel fairly confident that is where the problem is.
Mick

[BlindJoe]

Thanks, but I don't think that's it, smaller bore mc's are a fairly common upgrade on these bikes.  P=F/A, holding the force constant while decreasing the cross-sectional area of the bore means a greater pressure at the brake pad.

[mick7504]

That's correct.
A smaller bore master cylinder will deliver more pressure but less fluid displacement.
Whereas a smaller diameter piston in the caliper will deliver less presssure and will require less fluid displacement to operate.

[Kevin400F]

Is the new M/C a remote-reservior type, or can you remove the cap and very the "recouperation hole" in the reservior (nearest the hose) is clear and allowing fluid return?

If the hole is clear, try unbolting your caliper and pushing the piston all the way in to the caliper, which should chase any air bubbles upward through the lines and into the M/C reservior.  Make sure the brake line has a "continuous rise" with no pesky loops to trap air, and go slowly so you don't spurt brake fluid all over everything. 

You might have to siphon off some fluid to avoid overfilling the M/C reservior, but if there is air in the line this process ought to push it out.  You may have to repeat the process....pump the piston out to normal operating position, or slightly beyond, let things settle...tapping on the lines to encourage air bubbles to rise, then chase them up into the reservoir by pushing the piston in again.  Hope this helps.

[BlindJoe]

The stock bore size is 5/8" and you quite possibly aren't getting enough fluid displacement from the smaller master cylinder.
I feel fairly confident that is where the problem is.
Mick

Hey Mick you were right, I put the stock mc back on and the brake's working fine now. Obviously I wasn't taking the larger volume of the later calipers into account when reading about the smaller bore mc swap. I feel stupid for wasting so much time on it, oh well, lesson learned. Thanks

[mick7504]

The stock bore size is 5/8" and you quite possibly aren't getting enough fluid displacement from the smaller master cylinder.
I feel fairly confident that is where the problem is.
Mick

Hey Mick you were right, I put the stock mc back on and the brake's working fine now. Obviously I wasn't taking the larger volume of the later calipers into account when reading about the smaller bore mc swap. I feel stupid for wasting so much time on it, oh well, lesson learned. Thanks

That's good news.
They do get a bit temperemental sometimes but glad to hear that it's all good now.
Mick

[Don R]

Kevin suggested a good technique for opening the small "recouperation" hole, that's one to remember. I saw a very experienced Honda tech do that last week and it worked!

[JimJamerino]

The piston seal for my 1980 CB650C was supposed to be the same part number as the original post. It doesn't fit either. I'm going to try the suggested part number and repost if it fits.