New aftermarket front master cylinder/brake assembly fluid levels and stroke ?

[Guest]

Hi all,

I just swapped out an old leaking OEM front brake assembly/master cylinder for an aftermarket model.

While filling the new master cylinder I over-filled it slightly.

(the window to measure levels is on the front facing the road ahead instead of facing the rider.
Strangely enough the master cylinder has 2 lines for measurement- a "lower" level labelled LOWER and an "upper" level labelled LOWER)

After pumping a few times to build pressure (I have not yet bled the brakes yet) I find that pulling the lever only 1 inch seems to want to go to a hard stop.

Previously, with the older leaking master cylinder/brake assembly it took about 3 inches -about back to the bar to lock
The brake assembly I purchased off ebay is a brand new "xyb" model, that states it is "16mm or 5/8 bore" and stated being compatible with many gs models.

Will me correcting the fluid levels fix this issue ?

Was I just used to terrible brakes because it was losing pressure ?


thanks

sean

[tkent02]

Was I just used to terrible brakes because it was losing pressure ?

Sounds like this. If the lever ever gets anywhere near the bar there's a problem.

[seanarthurmachado]

OK I rechecked....it appears that 1/2 inch is the maximum travel.

I went out to the shed with a flashlight and tape measure. With the new brake assembly the handle is out fully (brake off) at 2 1/2".

When squeezed as hard as possible the lever is at 2.00" from the bar.

so only 1/2 inch worth of travel.

Is that normal ? It seems a little too close to me, but what do I know.

sean

[dantodd]

As long as the brakes are not dragging then it is probably right. You should be braking based on feedback pressure anyway.

[tkent02]

Not dragging and if they are braking as powerfully as they should. Just checked my bikes, new and old ones. They all go about a third to half of the way to the bar, none less than a third of the way, none more than halfway. Moving only 1/2" there's probably something not quite right.

Just thinking out loud here...

Your 16mm MC is a large diameter, it will pump a lot of fluid, but won't make as much pressure against the pads at a given finger pressure as a smaller diameter cylinder. Maybe it pumps too much fluid? I think the '79 850 had two front brakes? If it only had one that might explain why the lever doesn't go very far...

What are you using for brake lines?

And are the calipers free to move as they should? Pistons slide in the bores OK? Did you take them apart to look inside?

[seanarthurmachado]

Yes, the '79 has dual front brakes like the 1980 and newer models do. The only difference is that the rotors are plain and flat with no drilled sections and the calipers have the older round pads instead of the newer squarish ones.

The brakes were gone over and cleaned up by Steve, calipers stripped and all the gunk cleared out, brakes bled, new pads, fluid.

The brake lines are standard ones to the bike, the rubber ones. Nothing fancy like metal lines.

Since that time I discovered a leak in the front left caliper, and rebuild the front left caliper with new seals and piston. That fixed the leak on the bottom that had developed.

I was not seeing any leaks but the fluid seemed to be getting low, and I finally found a leak from the underside of the master cylinder/brake assembly.

I purchased a whole new assembly and installed yesterday, removing the old unit, placing the newer one on the bike, and filled with fluid today.

The unit is a cheapie model off of ebay, but claims to be compatible.

Sean

[seanarthurmachado]

I just thought of something else...

When I was attaching the new lever that came with the assembly I needed to push it against the piston to get it to mate up with the bolt hole. I just checked the old model with the OEM style brake lever attached (the '82 model I think) and it fits the old brake assembly with no pressure at all.

Maybe the new lever was a little out of spec. I'll try swapping levers tomorrow night and see if maybe the travel problem is due to the piston already being pushed in a little.

sean

[tkent02]

I just thought of something else...

When I was attaching the new lever that came with the assembly I needed to push it against the piston to get it to mate up with the bolt hole. I just checked the old model with the OEM style brake lever attached (the '82 model I think) and it fits the old brake assembly with no pressure at all.

Maybe the new lever was a little out of spec. I'll try swapping levers tomorrow night and see if maybe the travel problem is due to the piston already being pushed in a little.

sean

OK, thinking out loud again, if the lever isn't letting the piston come back all the way it's probably not opening up the return port, which can cause the brake to heat up and eventually lock.
This lever came with the new master cylinder? Odd that it wouldn't fit. Is this one of the super cheapos from China or a decent quality one?

[seanarthurmachado]

It's a Chinese model from "xyb" according to stamping on parts. (Like $25 or $30 for the box containing the assembly, lever, and a free rebuild kit for it.)

Sean

[tkent02]

Interesting, is there a manufacturing flaw keeping the lever from working properly? Maybe someone just threw the wrong lever in the box? Was something assembled wrong inside? Or maybe the whole thing is just a piece of crap?

I have used a several of those Chinese master cylinders, even a couple in the $20-$30 something range. They have all worked fine.

Got a link to wherever you bought it from?

[seanarthurmachado]

From eBay

http://www.ebay.com/itm/271183218282?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

[mmattockx]

OK, thinking out loud again, if the lever isn't letting the piston come back all the way it's probably not opening up the return port, which can cause the brake to heat up and eventually lock.

That is easy to check. OP, lift your front wheel off the ground and spin the wheel. Apply brake firmly and then let off the lever. Try spinning the wheel again. If it doesn't spin pretty easily with only a bit of brake drag then your M/C isn't letting the fluid back out into the reservoir and you need to correct that.


Mark

[seanarthurmachado]

OK I went out to the shed, removed the aftermarket handle and tried to replace it with the original (it sort of fit) but made no difference of the amount of pull on the lever to full lock on the brakes.

I think Ill be contacting the seller.

maybe just a bad unit or poorly designed one.

sean

[habsdoc]

Maybe you just did a really good job of bleeding the brakes and that is all the travel there is. I know my brake lever moves less than an inch before I have lock up the wheel pressure.

[seanarthurmachado]

OK I am returning this master cylinder/brake assembly.

(I rolled the bike forward by hand in the shed, touched the brake, and it locked with almost no pressure and the shed floor made a popping sound.)

Im not seeing any OEM units on ebay, and none noticed on here in the for sale.

what do people recommend ? Im seeing aftermarket units with 16mm piston (like the one I bought that is ridiculous) and aftermarket with 14mm piston. I think the GS big bikes are 15.XXX if I remember seeing correctly.

Does anyone have a recommendation ?