Brake reservoir' gasket leaks, alternatives?

The crusty buildup around the reservoir gasket race on the rear brake master cylinder was so thick that (after cleaning) when I put the old rubber O-ring back in it no longer filled the gap. Now it leaks/seeps fluid.

The master cylinder is rebuilt and works strong.

I am going to look at a plumbing hardware store for an O-ring that might fit.

Is using caulking/gasket sealer/silicon... an option? Worst case scenario is that I may need to go and get another master cylinder from a junkyard if I ever really need to pull the reservoir cup again...

Any thoughts on ways to cure the leak at the reservoir to master cylinder joining other than to find a rebuild kit?

hardware store o rings are going to be geared more towards sealing water,i think you might have better luck at auto parts store

I thought that there was a difference between brake fluid and petroleum based fluids. i.e. The rubber gaskets were damaged by the gasoline, where brake fluid does not degrade the rubber.

Therefore, won't rubber "for water" gaskets also work to seal up a reservoir that holds brake fluid?

I found an O-ring at the hardware store that I think will do the trick, I am going to go and test it on the bench now.

I guess and believe there will be no problems.

Anyone else know differently? I am working with brakes here and am always appreciative of the help.

Quinn

Qdude, Perhaps I'm missing something.every brake master res.I can think of uses a rubber diaphram to seal brake fluid from cap/and incorperated vent. what's the bike? G

Use DOT 5 brake fluid. It doesn't eat paint and it is compatible with all rubber stuff for sure.
It also has a very high boiling point (less brake fade), and doesnt absorb water as quickly.

Cheers.



PS. DOT 5 is not compatible with any other fluids. Hydraulic system must be completely drained first.

I'd stay away from DOT5. DOT5 brake fluid is not hygroscopic - it doesn't absorb moisture; so the moisture that enters the brake system will "puddle" at the lowest parts of the brake system and cause corrosion. Obviously, some folks here have used DOT5 with good results; maybe they're not in a high humidity environment, who knows. DOT5 has a very high boiling temperature and is often used in non-ABS racing applications, but racers change their brake fluid often thus purging the water. On a KZ650 with stock brakes the difference in boiling point is meaningless as you will never generate enough brake heat in street riding to boil DOT 3 or 4. If you stick with DOT 3 or 4 just be careful not to spill it on your paint. If you do decide to use DOT5 you will need to flush your entire brake system very thoroughly or you will experience compatibility problems, and you should flush and refill at least annually to mitigate the puddling problem. Ed

GANE-

If I understand it correctly, rubber is NOT impervious to petroleum. So, don't use rubber for a fuel line, the gas will eat it up.

Brake fluid does not attack the rubber, so it is used on brake systems.

Therefore, I think I am safe using plumbing O-rings as the gasket for the reservoir.

So far the fix works like a charm.

I am going to stick to Dot 3.

just need to bleed the front brakes now to finish the hydraulics.

Oh yeah, 77 kz650c1.

Qdude wrote:

The crusty buildup around the reservoir gasket race on the rear brake master cylinder was so thick that (after cleaning) when I put the old rubber O-ring back in it no longer filled the gap. Now it leaks/seeps fluid.

The master cylinder is rebuilt and works strong.

I am going to look at a plumbing hardware store for an O-ring that might fit.

Is using caulking/gasket sealer/silicon... an option? Worst case scenario is that I may need to go and get another master cylinder from a junkyard if I ever really need to pull the reservoir cup again...

Any thoughts on ways to cure the leak at the reservoir to master cylinder joining other than to find a rebuild kit?

Does the o-ring in question appear on this diagram?

Good Luck!

[Click on image to enhance view.]

Yea, there is no oring, so if there is one there it shouldn't be. #4 in the picture is the diaphram which provides the seal. You'll notice the cap of the master cylinder has a slot to let air get up through the threads into the center of the diapham. This allows the diaphram to drop and rise in the master cylinder to compensate for level changes from brake pad wear ect, and still keep the fluid sealed from moisture. You can't simply seal the cap to the body with no diaphram setup. We've talked about dot5 here many times. It's worked fine for me, but it's dry up here. I do the maintenence strickly by the book though and change the fluid every 25yrs :laugh: I've never had one caliper problem, and only one piston leak on a master cylinder in over 30 yrs.

o ring under resevoir im thinking

Ach,I'd forgotten that one, and miss-read as well. Sorry. But throw fuel on the fire,, Brake fluids do react to different rubbers, and in some cases changing from dot 3 to say dot 5 can cause deterioration/swelling. rare, but there. Be cautious using plumbing bits, & keep any eye on. luck G

Some reservoirs aren't removable.
Others are removable, with an o-ring underneath.
And some are remote with a hose connection.

Apparently the typical parts diagram fails to show whether or not the reservoir is removable.

As I recall, when looking down into a removable reservoir, two screw heads are visible, which screws hold the reservoir onto the bore portion, with an o-ring sealing the juncture.

As to front master cylinder on large fours, here's a pic and some info from Redline.
redlinecycle.com/Brk%20-%20Trap%20Rsrvr%20O-%20Ring.html

Fwiw, the 1976 KZ900-B1 LTD rear master cylinder doesn't have a removable reservoir.

Good Luck!