Rear Master Cylinder - partial kit, rebuild or replace?

[850GT_Rider]

Did a full rebuild of the rear caliper, now having a heckuva time getting it bled. Have a MityVac, and have cycled several bottles of brake fluid thru the system, but I'm still getting a consistent stream of micro-bubbles out of both bleed screws in the caliper, and the pedal is still spongy.

1) Can some kind of defect in the master cylinder allow it to entrain air, yet doesn't leak to the outside?
2) If (or I suppose WHEN) I decide I need to rebuild the master cylinder, is a just a seal-and-spring kit typically enough?
3) Given that the full "Master Cylinder Assembly" from Suzuki is over $150, are there other completely new cylinders that will work? Ebay shows a fair number of cylinders from manufacturers like Brembo that are less than the parts from Suzuki.

What says the collective?

 

[Nessism]

It's likely that the MitiVac is sucking air around the bleeder nipples. You may want to wrap them with some teflon tape and try again, at a low vacuum level. Or better yet, switch over to pumping the brake pedal after the majority of the air is out.

As far as brake system rebuilds are concern, you need to make sure the small master cylinder return port is open otherwise the system won't feed fluid correctly. Also, may times the master cylinder bore is pitted from corrosion which causes internal leakage. Before attempting to install a new kit into an old master you need to look through the master with a flashlight or similar, and make sure the bore is clean. It's getting tough finding good masters these days so good luck with that if the bore is indeed pitted.

 

[850GT_Rider]

Yeah - good point about the Mityvac. Although i did use thread sealant, its possible it produces too much suction.

Tearing into the master cylinder will absolutely be a last resort. Rear brakes were fair but workable. What started as a job to replace the shocks turned into a host of might-as-wells. New rear tire, spline lube, etc. Was just going to replace brakepads, found torn dust seal. Got new seals, opened the calipers up, found pitted pistons, and inboard piston was frozen. Ended up with full rebuild.

Will see if pedal-pumping solves it. Here's hoping.....

As to replacement, I would have thought by now that someone here would have adapted a non-OE rear master cylinder. Just looking for alternatives.

 

[tkent02]

Got new seals, opened the calipers up, found pitted pistons, and inboard piston was frozen. Ended up with full rebuild.

Yeah, you need to open up the master cylinder and clean the whole system thoroughly. A tiny flake of this rust in the wrong place can prove disastrous, as in the wheel locking and staying locked even if you haven't touched the brake pedal at all.

 

[850GT_Rider]

Yes, I know. Gotta do it right. Thx

 

[850GT_Rider]

Yeah, you need to open up the master cylinder and clean the whole system thoroughly. A tiny flake of this rust in the wrong place can prove disastrous, as in the wheel locking and staying locked even if you haven't touched the brake pedal at all.

Pulled the master cylinder and took it all apart. You're right - there's some VERY small passages in there. This is what I found at the bottom of the port where the line from the reservoir comes in:

 

[850GT_Rider]

After cleaning, and using a very fine welding tip cleaner to open up that second port.

Main bore looks clean, not pitted. Looks like I'll just need the piston, spring and seals kit. View attachment 41188

 

[Nessism]

Your brake line is all full of the same crud that's fouling that port. Hope you changed that too.

 

[850GT_Rider]

Absolutely. I had done everything EXCEPT tear down the master cylinder. Don't know why I thought it was ok. D'oh! :stupid:


EDIT/UPDATE: I had a machinist at work look at the cylinder, and there are a couple of very small pits and minor scratches along the bore. Barely visible, but they're there. Since I don't know IF they're a problem, or IF they can be honed out, or IF all that will result in a fully functioning cylinder, I'm just going to order a complete OE unit from Suzuki. Too many IF's for something this important. Replace it, bleed it, stop worrying about it.