Author Topic: MC differences - fresh install no good (Read 1682 times)

Nortstudio · « **on:** May 16, 2011, 09:02:07 PM »

I have 2 master cylinders, one from a 550 (seized - not sure of the year) and one from what I think is a 1976 750f (from a parts bike that was mangled and modified, so not positive of the year). One has the number 21 inside, the other 14.

I am hoping to put one on my '76 550k, and obviously the unseized 750 one would be easier. But I am not sure whether they are appropriate for my bike. They appear to have the same reservoir size, but that's all I've measured.

Can some one tell me what I should be checking to run one of these safely with my stock caliper on the '76 550.

Thanks,

Scott

scottly · « **Reply #1 on:** May 16, 2011, 09:23:17 PM »

Check the MC piston diameter (or the rebuild kit part #). If they are the same, go for it! As long as it doesn't leak and the passages are clear, it should be safe.

vrenlos · « **Reply #2 on:** May 16, 2011, 09:25:42 PM »

Ditto, and I believe that they should both be 14mm pistons - should be just fine as a replacement.

Nortstudio · « **Reply #3 on:** May 16, 2011, 09:26:52 PM »

Thanks folks

Nortstudio · « **Reply #4 on:** June 17, 2011, 01:15:19 PM »

So I finally got around to doing the full brake replacement today. Its got more movement at the handle than it should. Have to say, I was hoping for better results: maybe you can check my procedure, and see where I've gone wrong....

So I ended up going with the 750 MC, it was in much better condition. Rebuild kit went in fine. I gathered a new caliper piston from forum member who makes the new phenolic (sp?) type here, because the old one was pretty pitted. Got some steel braided brake lines (also made by member here). The pads/gasket were new in the fall, so I kept them, just made sure they were clean.

Bled the old oil out of the system, made a mess, installed new piston, installed new brake lines and the new (stock style) MC on the bars. Put new brake fluid in, and started getting it going. Bled the line based on Two Tired's procedure in the FAQ.

Unfortunately, it's pretty soft. My previous MC was an aftermarket type, in pretty rough shape. It has no logo or BR ad on it, so I never knewnhow to get a rebuild kit for it. But as rough as it was, and with the old rubber brake lines, it still had a tighter action to the lever.

Anything obvious I should check? I'll bleed it again tomorrow, to see if I missed some deep seated bubbles. I took my time. But I'm hoping that's it.

Is it possible I don't have the brake lines lined up correctly? Is there some way for the small holes to be misaligned, so that it's just not acting as it should?

Thanks in advance.

scottly · « **Reply #5 on:** June 17, 2011, 07:42:44 PM »

Proper bleeding procedure:
1) Position the caliper so the bleed screw is at the highest point. Place a piece of clear tubing on the bleed nipple feeding into a container to catch the fluid.
2) Pump the brake lever several times (make sure you have the cap on the MC), then, while maintaining pressure on the lever, crack open the bleeder. Look for bubbles in the clear tube. When the lever reaches the handle-bar, close the bleeder before releasing the lever. Do this until there are no bubbles, refilling the MC every 2-3 cycles. It's much easier if you have someone help operate the lever, but I have done it by my self.

Spanner 1 · « **Reply #6 on:** June 17, 2011, 08:02:18 PM »

There are 2 piston lenghts..... you said you changed the piston. Compare the new piston with the old one ( hope you still have it ! ) it may be shorter. The short piston sits too far back in the bore and pressure on the lever/ fluid just escapes up the return hole to the reservoir until the piston moves past the return hole it's not making any pressure in the system = spongy handle...

I'd check that first. I found this out myself on a re-build kit, couldn't get why I had a spongy handle, tried everything, took it apart again and just happened to lay the new piston beside the old one.

Nortstudio · « **Reply #7 on:** June 17, 2011, 08:04:43 PM »

Thanks scottly,

I did the procedure, but it was at the end of the full install, so I may have missed a few bubbles. I'm going to do it again in the noting - hoping that will be it.

This is the first time I have done it will a completely fresh install of all the components, so I'm sure I just have bubbles trapped in there. Maybe sitting overnight and riding it a little on the way home from the garage today loosened a few up.

Fingers crossed....

If not, I'll check the length of the piston in the MC. That would really suck!

Spanner 1 · « **Reply #8 on:** June 17, 2011, 08:16:16 PM »

Nort, just unscrew the res. lid and pull the brake, if you have the short piston you should see a spurt of fluid into the res. thru' the teeny return hole..... good luck

Nortstudio · « **Reply #9 on:** June 17, 2011, 08:19:08 PM »

I think that may have happened while I was bleeding it. I'll check it again tomorrow. Is there any way that I can request the proper lenth, if I need to ask for a new one.

I got this from bike bandit, and they have the whole procedure of ordering it for your specific bike. So I might need a part number???

scottly · « **Reply #10 on:** June 17, 2011, 08:32:50 PM »

Scott, any idea what the diameter of the aftermarket MC piston was? If it was larger the the stock 14mm, the "sponginess" may be a matter of perception. If this is the case, try the brake under actual riding conditions.

Nortstudio · « **Reply #11 on:** June 17, 2011, 08:44:08 PM »

I Don't know the diameter. I never measured it. It went it pretty well, but maybe that wouldn't tell me if it's right or not.

I rode it home today from the garage (12 blocks or so). The lever is definitely different. Pulled closer to the bar. It did stop though, so that's something

I'm afraid that the piston may be too short, based on Spanner's description. I definitely had some brake fluid shooting back into the reservoir when I pulled the lever.

I know I'm going from an aftermarket MC to this new (old style) one, with new braided hoses and a new caliper piston, so I really have no frame of reference.

But I certainly expected it to be be "better.".

Does anyone know the actual length it's supposed to be - the longer one?

What the hell is the shorter one for???

scottly · « **Reply #12 on:** June 17, 2011, 08:57:57 PM »

Look at the opening in your old MC where the piston goes in. Does it look bigger than the Honda?

Nortstudio · « **Reply #13 on:** June 17, 2011, 09:14:32 PM »

I'll check it tomorrow and report back. I'm just more pissed about the potential wrong size piston from bike bandit right now. They frustrate me....

scottly · « **Reply #14 on:** June 17, 2011, 09:20:23 PM »

Even with the correct length piston, some fluid should squirt from the relief hole in the first portion of lever travel...

Nortstudio · « **Reply #15 on:** June 17, 2011, 09:34:59 PM »

I'll do a new bleed, and see where that shakes out.

For the record, in a search of eBay, this MC looks to be exactly what was put only the PO.
http://cgi.ebay.com/ebaymotors/1983-Honda-Magna-V65-1100C-Front-Brake-Master-Cylinder-_W0QQcmdZViewItemQQhashZitem4aabd76e38QQitemZ320710602296QQptZMotorcyclesQ5fPartsQ5fAccessories#ht_500wt_948

Spanner 1 · « **Reply #16 on:** June 17, 2011, 10:32:26 PM »

Scottly, have to disagree about the piston vs. fluid return in reservoir. With the correct piston for the m/cyl. your using, the piston seal sits right at the return hole and sweeps past it with the first 1/4 inch of lever travel allowing all further lever movement to compress the fluid in the brake......... should be no fluid spurt into the reservoir........

scottly · « **Reply #17 on:** June 17, 2011, 11:22:24 PM »

Quote from: Spanner 1 on June 17, 2011, 10:32:26 PM

Scottly, have to disagree about the piston vs. fluid return in reservoir. With the correct piston for the m/cyl. your using, the piston seal sits right at the return hole and sweeps past it with the first 1/4 inch of lever travel allowing all further lever movement to compress the fluid in the brake......... should be no fluid spurt into the reservoir........

I agree, the spurt occurs only during the first portion of lever travel, until the piston moves past the return hole.

Nortstudio · « **Reply #18 on:** June 18, 2011, 07:17:08 AM »

Ok. I'm finally down at the garage (late start). First thing, I checked the "spurt" in the MC. It does do a little
Spurt from the left (small) hole - but only in the first little bit of lever travel.

I realized I had taken a picture of the piston before installing it - although not scientific, I also took a shot of the one that came out of the stock MC when I bought it. I guess they could both be the wrong, shorter kind, but here are the pics.

I'm going to bleed again. It didn't feel too bad riding here - but definitely not rock hard.

Nortstudio · « **Reply #19 on:** June 18, 2011, 08:22:32 AM »

The first photo in that last post is wrong. That was from the other MC I had. The one I used had a different piston in it - and at least based visually, the one I used (2nd pic) and the one that had been in it - they are the same.

I bled it again, and it seems better (there were definitely some bubbles - but not many). I'll ride it a bit and see if I can get used to it. I forgot to mention that I definitely feel more dive from the forks with this new set-up. So maybe thats a good indicator. Now to do the fork overhaul

The old aftermarket MC I had on there actully did have a little info. It says 5/8 on te bottom, not sure how that would effect the differences I'm noticing.

Thanks for all the help folks.

scottly · « **Reply #20 on:** June 18, 2011, 09:46:22 AM »

Yes, the 5/8" (15.875mm) MC would make for a very short stoke, firm lever. Those are used on dual disc applications. Also, it doesn't take much air at all to make for a spongy feel.

Nortstudio · « **Reply #21 on:** June 18, 2011, 10:06:14 AM »

Thanks for all your help. I think it's pretty good for now. I'll ride it and get used to it in real world circumstances, instead of obsessively pulling the lever while sitting still in the garage

Now onto the next fix....

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Author Topic: MC differences - fresh install no good (Read 1682 times)