2 Rear Master Cylinders, neither works - advice?

[Shuriken]

I have 2 rear master cylinder assemblies and neither works.

I tried a K&L ripoff kit - new o-rings, etc., and the rebuilt one still doesn't work.

I get a little pressure and suction out of them both, but perhaps there is air leakage somewhere? Can someone tell me the correct way to get these things working?

Should I get another K&L kit or try to get o-rings from Yamaha?

Do they still make the rubber peice that fits under the plastic cap?

What else should I be checking?

Ben

[ratbyk]

Bleeding Rear Master

First, bleed your best cylinder on the bench. Put it in a vise completely assembled with fluid in it. Push in on the push rod with your hand, once bottomed put a finger tightly over the hole where the line connects to. Allow the push rod to return SLOWLY. Pause for a moment, then repeat. 5-6 pumps like this should produce pressure. Bleed a couple of times more to ensure all air is expelled from the master. Hook the line up to the cylinder with the cylinder still off the bike. Now, bleed it again, hand pushing the push rod, opening the rear caliper bleeder screw. You can leave the bleeder screw slightly loose, place your finger over the hole in the screw tightly on the return stroke. Do not hurry your pumping action. Slowly and smoothly push in and let return slowly. Pause for a moment between each stroke to allow the master bore to refill from the reservoir. If you hold the master more horizontal than it normal vertical position you will also have a better chance of purging all of the air from the bore of the master. Works for me every time.
Don't touch your eye with brake fluid on your hands.Avoid all contact with painted surfaces..
Ken/Sooke

[Shuriken]

So I suppose the bleeding will be in vain if for some reason there is not enough pressure to do what you say....

I wonder why a new set of rubber o-rings doesn't work. Where else would these things get air from, and wouldn't they also leak from the same place?

ben

[xssiveone]

Originally posted by ae7f
I wonder why a new set of rubber o-rings doesn't work. Where else would these things get air from, and wouldn't they also leak from the same place?

ben

They get air when you have them apart, or anytime you have the sealed system open. What Ken has described is standard practice with master cylinders, doesn't matter whether it is on a motorcycle or a car. You're trying to bleed the system, when the master cylinder is still full of air. It draws the same air back in from the lines that you forced out. What Ken has described forces the chamber to fill with fluid when the piston retracts, as it can't pull the air back in.

[ratbyk]

Master Cylinder Problems

If you cannot create pressure while bench bleeding you certainly will not get pressure with the cylinder on the bike.
Possible causes;
Mis-assembly of the order or direction of internal parts.
A scored master cylinder bore.
A master cylinder bore worn oval.
A cut or turned lip edge on one of the rubber cups
I have encountered cylinders over the years that because of bad rubber cup lips/edges would allow air to bypass on the return stroke. This would continually introduce air into the system as fast as you bleed out the air introduced in the previous cycle.
If you cannot create pressure while bench bleeding it is time to disassemble and start over.
Ken/Sooke

[Shuriken]

I'll definitely give it another try.

The parts are assembled correctly. I examined a spare from another bike to verify assembly order.

Thanks.
Ben

[Geezer]

Originally posted by ae7f
I'll definitely give it another try.

The parts are assembled correctly. I examined a spare from another bike to verify assembly order.

Thanks.
Ben

Often old worn or bad brake seals with look almost new when cleaned up but won't work worth crap. You may have gotten a bad set of seals.

Geezer

[Shuriken]

Perhaps. I was a victim of the K&L curse.

Ben

[nashville_bill]

Do it backwards!

One trick I used with MGs and Triumphs was to fill a clean "Pressure fed oil gun" (the can that you put oil into and pump it out using a trigger to lubricate things) and connect it to the bleeder on the caliper. Open the bleeder slightly and pump the fluid from the oil gun through the caliper up into the master cylinder. Be careful to start with an empty master cylinder. After you fill up the master cylinder, you can bleed the brakes as you would normally.

[randy]

OK, while we’re on the subject of brake bleeding tricks, here’s one I got years ago from my dad.

Take a big old empty glass pickle jar. About a gallon size. Must be glass with a metal cap.

Pop two holes in the cap and epoxy old bleeder nipples in each hole. Grease the cap seal and screw it on tight. Attach two hoses, one goes to a vacuum port on the engine, the other on the caliper to be bled.

The engine does the work and the jar catches the old fluid. I’ve used this gadget on many cars over the years to do complete system fluid changes. Used it on my XS, it pulled fluid so fast the air bubbles didn’t stand a chance. No sore hands from pumping, either.

I’ll be using it again this weekend after I re-seat my caliper piston seals. AARRRGH!

[Shuriken]

Wow. This turned into a good thread.

I spent a lot of time last night using Ken's method and got one of the master cylinders to work. It's nice to have rear brakes again.

I don't know if I will be able to use the other assembly because when I bought the K&L rebuild kit and tried to use the plunger that came in the kit, it was too large and I don't think I did anything but trash the bore.

Ben

[ratbyk]

Brakes Return

Glad I could help. Which of my tricks worked for you?
Ken/Sooke

[Shuriken]

Your above post at: 07-12-2004 05:49 AM "Bleeding master cylinder" was the one I used.

I spent a lot of time going slow and repeating. I thought my time and parts were again at a loss until I pushed the brake lever down as far as it would go and felt some response.

I also adjusted the pushrod underneath the plunger.

Ben

[ratbyk]

Rear Master Push Rod

Be careful not to adjust the push rod to far in. If you do on the return stroke the primary cup will not uncover (expose the compensating port-spooge hole). If this happens the expanding brake fluid from heat cannot escape back into the reservoir. Pressure in the system will build up to the point at first the brake will drag. This will compound into more heat, more expansion-locked back wheel.
If this does hap[pen to you simply open the bleeder screw for a moment to let the pressure off. Watch your eyes it could spit out with some force. Then adjust the push rod so that it is slightly shorter.
Ken/Sooke

[ratbyk]

Push Rod Adjustment

ae7f, if you have a Clymers manuel the proper sequence and settings for the master cylinder push adjustment are covered on page 38.
Ken/Sooke