Bad front Master Cylinder?

[Guest]

I rebuilt both master cylinders, built new lines, soaked, rebuilt and painted the calipers on my bike. I had a heck of a time getting the rear MC put back together in the correct order, but got it anyway and the rear is working great. The problem is the front. I know I am putting it together right since there are less pieces and I paid attention when taking it apart (something I didn't do too well on the rear). I have been able to get fluid to the calipers, but I am not able to get the system to build pressure. I am wondering if this is caused by an incorrectly sized rebuild kit, or a bad master cylinder. I am not sure, but there could be pitting in the cylinder, I cant see too well in the bore, but wouldn't be surprised. Another possibility that I thought of is maybe the rebuild kit is the wrong size? The circlip that it came with was a little undersized for the master cylinder.

If I am not able to rebuild this one, I am seriously considering replacement. I know there a lot of threads about replacing with MC's off of other UJM's but would like to keep it as close to original looking as possible as well as safe.

[Nessism]

Did you rebuild the calipers as well? If so did you use OEM parts or aftermarket? Reason for asking is that K&L brake system parts are garbage and can cause the issues you mention. The caliper seals are too tight and don't allow the pistons to move close enough to the discs which results in a spongy brake level feel. One other possibility is that the master is not priming properly. If the small return port is plugged fluid won't fill the pumping chamber properly. Sometimes you need to force fluid into the system (after clearing the small hole) and then the master will start pumping.

Good luck

[roeme]

I rebuilt both master cylinders, built new lines, soaked, rebuilt and painted the calipers on my bike. I had a heck of a time getting the rear MC put back together in the correct order, but got it anyway and the rear is working great. The problem is the front. I know I am putting it together right since there are less pieces and I paid attention when taking it apart (something I didn't do too well on the rear). I have been able to get fluid to the calipers, but I am not able to get the system to build pressure. I am wondering if this is caused by an incorrectly sized rebuild kit, or a bad master cylinder. I am not sure, but there could be pitting in the cylinder, I cant see too well in the bore, but wouldn't be surprised. Another possibility that I thought of is maybe the rebuild kit is the wrong size? The circlip that it came with was a little undersized for the master cylinder.

If I am not able to rebuild this one, I am seriously considering replacement. I know there a lot of threads about replacing with MC's off of other UJM's but would like to keep it as close to original looking as possible as well as safe.

Funny, my situation is somewhat reversed: Got an universal front MC, rear MC still stock. All brake calipers rebuilt with new seals, but master cylinders untouched. Now the rear is acting weak. pffff....

But it's possible I made some mistake or didn't bleed enough. I'll see soon.

Edit: Or had to do any of the things Nessism mentioned, but didn't. Good to know. (Taking notes)

[jdion81]

Oh yeah, for got about that, I did rebuild the calipers, with new pistons, seals, and dust covers. These were all from suzuki. I did run a small piece of copper wire through the two holes while I had the MC on the bench. Would there be fluid at the calipers if the MC was not primed?

[jdion81]

For an aftermarket, what do you think of this one? http://www.tcbroschoppers.com/black-...lebars-rh.html

[Nessism]

For an aftermarket, what do you think of this one? http://www.tcbroschoppers.com/black-...lebars-rh.html

No good. You need a 5/8" bore master (or maybe 16mm).

[Skinner]

Assuming you have the correct re-build kit your problem could be that the snap ring is in the incorrect groove. There are two grooves, the outermost one being where the dust seal fastens. The snap ring goes into the innermost groove. It can be a bugger to get the snap ring pushed down far enough to properly install, speaking from experience. I had my MC rebuilt, reinstalled and filled with fluid only to have it leak all over the place - the snap ring was installed in the outermost groove.

May not be your issue, but it's worth a look.

[jdion81]

I'll take a look at that. I'll also try to get some photos.


Any thoughts on pitting in the bore? I assume this can be bad.... Again, not too sure on whether there is pitting or just discoloration.

Assuming you have the correct re-build kit your problem could be that the snap ring is in the incorrect groove. There are two grooves, the outermost one being where the dust seal fastens. The snap ring goes into the innermost groove. It can be a bugger to get the snap ring pushed down far enough to properly install, speaking from experience. I had my MC rebuilt, reinstalled and filled with fluid only to have it leak all over the place - the snap ring was installed in the outermost groove.

May not be your issue, but it's worth a look.

[Skinner]

I'll take a look at that. I'll also try to get some photos.


Any thoughts on pitting in the bore? I assume this can be bad.... Again, not too sure on whether there is pitting or just discoloration.

Well, I can't speak to pitting in the bore, but I can speak to the inability to build pressure in the system. I just completely re-built the front brakes on my '82 Katana. I could not get pressure to build in the lines for the life of me. In retrospect I should have backfilled the system to begin with, but hindsight is 20/20. Instead I simply filled the MC and started pumping. It took forever to get fluid through the system, and when I did finally manage that, I could not build any pressure and, as it turned out, there was alot of air. I finally bought a vacuum pump and used it to bleed the lines. I also cracked the banjo bolt at the MC and pumped the lever. I loosened it just enough to allow any trapped air bubbles to escape. Success! I now had pressure in the system and all the air bubbles were gone. I then pumped up the pressure and strapped the lever to throttle for about 24 hours. I now have a firm brake lever.

Again, this may or may not be your issue. However, I know that I went about filling the system with brake fluid the wrong way for a complete re-build. Could be the same for you.

[GSXR7ED]

Pitting inside the MC? I could see that if the Piston had to be manipulated out of the chamber. Otherwise, a good soaking will reveal tiny holes that allow some pressure to be released during the application of brake.

The toughest part is getting the trapped air out of the line but using a pressure tool to bleed and of course loosening caps allows for air pressure release.

I'm glad you figured out the problem. It does test your patience but once figured out, you have confidence in your brakes when you need them.


Ed

[jdion81]

Does the angled reservoir mean anything? I found this master-cylinder, but it says it doesn't have an angled Reservoir.

[Nessism]

Does the angled reservoir mean anything? I found this master-cylinder, but it says it doesn't have an angled Reservoir.

Either the photo is wrong or the description is. The photo shows a "14" which means 14mm.

[roeme]

Does the angled reservoir mean anything? I found this master-cylinder, but it says it doesn't have an angled Reservoir.

I guess if you have clubman bars, or any handlebars that point down...you'll have to account for that with common sense (H/L now longer reading properly, hole into master cylinder not above the "waterline"...).

Of course, depending on the reservoir, they cannot be tilted beyond a certain degree. It's why quite a few lever styles have the reservoir separate from the MC. See my 550 rebuild, where I have such aggressively angled handlebars, and such a lever on them. Fits this particular bike very well (and at some point I will replace the reservoir with something nicer, but it's ok for now).

The bike had stock levers previously though, and must've been working fine. That reservoir ain't angled as well, as per my understanding.

[jdion81]

Either the photo is wrong or the description is. The photo shows a "14" which means 14mm.

Yeah, that is weird, one of the pictures has a 14 in the casting, but then has 15.8 printed at the top of the reservoir near the lid. I went ahead and bought one, I figured $50 is worth the piece of mind of having a good braking system.

BTW, I just stopped at home on lunch and pulled the hose off of the current MC and pumped the lever while holding my thumb on the end and it pumped up pressure. Maybe some air in the lines still?