Front MC question

Wingedwheel

If it wasn’t broke before, let me try it…
Top Contributor
Messages
2,564
Reaction score
5,386
Points
513
Location
Waterford, MI
When I built my ‘76 I cleaned and powder coated the MC body and installed the standard rebuild kit. On a few occasions when I squeezed the lever I had no pressure.It is random and although it is alleviated by another quick squeeze I don’t want this to become an issue. Obviously I’m losing pressure but what should I look for or should I just get another $20 kit? I have other MC’s I could use as well but they possibly would need a kit also. Gearing up for spring!
 
Sounds like you've got a bubble of air in the line.
Try more bleeding of the system.
There are numerous threads about bleeding procedures.



.
 
I really never thought about that. I got an email that lower brake lines were back in stock at 650 Direct. Maybe I’ll just do a total hose replacement and bleed. Thanks Dude
 
I rebuilt the caliper but I have to use the two-piece line set-up because of the brake switch. Nobody had a lower line in stock At the time.
 
Old rubber brake lines can internally collapse or start shedding rubber "slivers". Have run into both those issues before...........
It's also possible to block the equalization port in the MC with handle limit screws set out of range.
 
I’ll look up that switch 3g thanks. I think the last time I visited the hose issue it wasn’t mentioned. since you mention the screw I think it’s in too far however it’s set according to handle squeeze and if I back it out I have no lever. I have to wonder if anything was blocked wouldn’t it do it every time not randomly?
 
I looked at those switches and they are quite reasonable. I wish there was a better description of the length of the wires
 
I ordered the replacement rubber and steel lines for the 1976 model. Used 650 direct, I think that they are the same as Mikes XS. Just located here in Canada.

If the original rubber ones lasted for 47 years, what the heck, these will probably last until it is definitely not my concern anymore.
 
I’ll look up that switch 3g thanks. I think the last time I visited the hose issue it wasn’t mentioned. since you mention the screw I think it’s in too far however it’s set according to handle squeeze and if I back it out I have no lever. I have to wonder if anything was blocked wouldn’t it do it every time not randomly?
That's a big statement.
Do you notice any difference if the brake gets hot, say a couple strong applications in a row?
There are two ports in a master cylinder the main port it's fairly big say 1/8" and the equalization port, it's tiny, not much bigger than a single strand from a copper MC motorcycle wire. Setting your screw "in" quite a ways tends to block that port. You might be right on the edge of blocked or not.
I don't have a hard answer for you about what's going on but as others have mentioned air trapped in the line up at the MC banjo bolt or inside the caliper are favorite hiding places for some air. One thing I like to do is place the bike on the side stand, turn bars full stop left, the idea isgetting the MC higher than that MC banjo bolt. Back off that handle bolt, open the cap, squeeze release watch for bubbles repeat til they quit. See what you have.
There's other bleeding "tricks" too.
One is to remove the caliper, place it so the brake lines run ONLY up hill from caliper to MC, (NO low spots or dips) force BOTH caliper pistons back all the way. You may need to remove brake fluid form the reservoir so it doesn't over flow while doing this pump them back out with the lever added fluid if needed, maybe repeat then see what you have.
 
Well I just looked and they are out of the upper. Mikes has both but not the spring covered lower.
 
Gary it doesn’t matter if it’s hot or cold. Once it did it when I first got on the bike and other times it’s been after riding it for awhile.I never adjusted that screw since it appeared to be where it needed to be. I did try and back it off because it seems bottomed out but then like I said there was no lever. I’ll delve into it this week as I also need to do a few other things but at least I have things to look for.
 
Have you considered changing out the MC for a slightly smaller one? Yamaha used sizes that really were a bit too big back then. I'm pretty sure yours is a 5/8" bore. The later MC with the 14mm bore is supposed to work well with your older type 2 piston caliper .....

MC Chart.jpg
 
I have read that in other posts and it’s a consideration. If I go with the one piece line what length should I order?
 
Back
Top