Rear master cylinder

[Saddle_Bag]

Tech question, I have discovered that the rear master cylinder needed to be rebuilt. I decided to remove the reservoir and clean it out. I have never touch this but to add fluid as I bleed the system. There was crude in the tank, but I had one heck of a time getting the lid off of it. The previous owner, back in 1989 somewhere there abouts, took it apart and stuffed the rubber diaphram down into the reservoir. I don't think it will ever get back into a usable shape again. Is this needed for the system to function properly?? Oh, forgot to add, the reservoir was glued shut with something. I did manage to get it open, and I didn;t brake it. Do I need it? Or is there something that will work? I did see that there is a groove in the lid and the top of the reservoir for venting I guess and to allow the diaphram to expand as the fluid level drops. Will a thin rubber gasket work? joe

[MiCarl]

That diaphragm moves down with the brake fluid, the vent you see lets air on top so the level can go down.

 

If you don't have the diaphragm air will contact the brake fluid. The brake fluid will absorb moisture from the air and be ruined.

 

Of course that diaphragm is discontinued. You can replace the whole master cylinder assembly for about $513.

 

Best to watch eBay for one. Also, could rig some other reservoir as a replacement.

 

Are you sure the old one is deformed? It should accordion back into shape.

[Saddle_Bag]

The thing had been push in and kinda twisted. I have it on the deck in the sun trying to get it in some type of shape. I have a small piece of plexi glass sitting on it, but it is so twisted. I used plexi so that I could see the gasket area , I can't get it to even lay half way flat. According to Rich (Buckeye)the whole unit is not available. Even if it were , thats alot of mooula. How about contact cement to seal it in place? I don't know. Been one thing after another. joe

[Saddle_Bag]

MiCarl, I just got off of the phonewith a friend who has an 86 VR. He did a quick measure and thinks his resevoir is the same size as mine. He said his master cylinder is a larger size but all the rest is the same. I checked the parts list and they carry different numbers. Do ya think mammy changed the numbers but only changed the mastercylinder in 86? joe

[Fannydunk]

Saddle Bag, I just finished rebuilding my 89 VR. I got that rubber piece from my local dealer. If the same part fit yours they are still available. My was as yours, so out of shape it would never fit back in. Also I got a master cyclinder rebuild kit. It was about $55.00

[GeorgeS]

If you use Synthectic brake fluid, or DOT-4, you don't have to worry about Moisture.

 

However, I can't say if leaveing out the Rubber thing will really make any difference.

 

 

I use Valvoline Synthectic Brake fluid in Everything, and have for years.

No more DOT - 3 for me

[MiCarl]

MiCarl, I just got off of the phonewith a friend who has an 86 VR. He did a quick measure and thinks his resevoir is the same size as mine. He said his master cylinder is a larger size but all the rest is the same. I checked the parts list and they carry different numbers. Do ya think mammy changed the numbers but only changed the mastercylinder in 86? joe

 

I saw that the part is still available for the 86+ models. My experience is that Yamaha doesn't change part numbers unless they change the part.

 

The cap on the later models also has a different part number.

 

Now, the changes may be superficial or they may be significant. You might get away with a later diaphragm, a diaphragm and cap, or it might be hopeless. Only way to judge will be to get hold of the parts and compare them.

 

Good luck!

[Gearhead]

If you use Synthectic brake fluid, or DOT-4, you don't have to worry about Moisture.

 

 

George, I beg to differ here. Not about the synthetic, that I don't know about. But DOT4 is the same as DOT3 except with a higher temp rating. Both are supposedly equally hygroscopic (they absorb water).

 

BTW, hygroscopic is a good thing! I've read that some brake fluids, DOT5 methinks, is not nearly so much so. This is often thought of as a good thing, but I've read otherwise. Moisture gets into the system, one way or another. Condensation from when the cap is off, whatever - it gets in there. If the fluid is hygroscopic, it absorbs the water and TO A POINT is does no harm. That's why we should flush the hydraulics from time-to-time, esp. when the fluid is brown. But if the fluid is NOT hygroscopic, the water can condense in the caliper, rusting the piston.

 

Jeremy

[Saddle_Bag]

I'm going to order the 86 part. I hope MamaYamaha doesn't let me down. I'm hoping the same part, with a new number. If not, DOT 5???? joe

[Snaggletooth]

Hey Saddlebag,

 

As for that diaphragm, ya gotta try this. I had rear brake problem when I bought my '84 and the diaphragm was a messed up ball. I bought a master off ebay and it wasn't much better.

 

I took some parts down to a friend of mines auto shop to use his solvent tank to clean them up and he just laughed at me. He had a new toy. He had a high pressure, heated solvent cleaner system. It sprays heated solvent at pressure in a closed tank and it really does a nice job. We tossed all my stuff into the basket and at a whim I tossed both diaphragms in with everything else. What the hell, what could I lose?

 

When the system shut down all the parts were spotless and dry. I dug through the parts expecting the diapragms to be toast. Not so. They both had recovered their original shape and looked like they just came of a factory bag. Soft and flexable. I've had one in my rebuilt master now since early last season and it's doing fine.

 

If you can find a local shop with a sytem like this, and they are comon these days, it's worth a shot to ask if they can throw it in and see what happens. It sure doesn't seem to hurt them.

 

Mike

Edited July 31, 2008 by Snaggletooth

[Saddle_Bag]

Hey thanks, I have been tied up work at a coin show in the area. I'll take it out tomorrow and see what I can find, cleaner wise. joe