RD400 rear brake master

SoCal250 · February 24, 2020, 11:13:37 PM

Two part question:

1) Any one have a good solution for a replacement for the OEM RD400 rear master? Looking for alternatives in case I can't get mine sorted out. I'd prefer something relatively simple that doesn't require a lot of fab work. I've seen the Magura unit at HVC, however it seems that the rebuild kit might be an issue someday since the're already kind of hard to find.

2) I just rebuilt my rear master for the 2nd time in the last couple years. The previous rebuild kit started seeping past the seal and oozing fluid out of the cylinder while parked in the garage. I don't recall if that kit was OEM or aftermarket. I don't think it lasted more than 12-18 months. I just installed a new OEM master kit after completely disassembling and cleaning it. The issue now is that I can't get the master primed or pumping fluid, I even left the pedal depressed overnight to see if fluid would drain down into it. No luck. Any suggestions before I remove it and start all over? Thanks

Organicjedi · February 24, 2020, 11:24:28 PM

Have you tried cracking the banjo bolt and seeing if you are getting fluid into the MC in the first place? Sometimes air blocks fluid from even getting to the MC.

You can crack the banjo and bleed it at the MC. When fluid starts seeping out of the MC and all the air is out, then tighten the banjo bolt and proceed bleeding the line.

This exact thing happened to my 400 after it was rebuilt.

2smokenNick · February 25, 2020, 12:41:45 AM

I made an adapter for my eBay special master cylinder since the bolt holes were off a tad. I'll have to see If i can find and upload some pictures later. Works really good. Maybe too good....

1976RD400C · February 25, 2020, 07:17:29 AM

The only way I ever got one to bleed the air out is pump fluid into the caliper bleeder valve and push the air up and to the master and out to the reservoir.

2smokenNick · February 25, 2020, 10:15:05 PM

I completely rebuilt my OEM rear master cylinder, honed it, the whole 9 yards but it just wouldn't develop any pressure. Tried sucking fluid from the caliper banjo bolt end to make sure it was well bled and still nothing. I've heard the rears pit really easy and don't and up sealing, which is exactly what happened to mine. It just leaked around the push rod boot. I was pretty frustrated to say the least.. So instead of buying an expensive magura or brembo unit, I had simply had enough and got creative!
Not the best machine work, but at least I'm not playin' video games like others my age...
;D

triple1972 · February 26, 2020, 12:22:07 AM

If you are not doing an all out resto is it better to just buy the re-production calipers? I actually mean that as a question nothing more. I found a repop one that was aluminum for 95$. Hope it's a good purchase! I have never rebuilt an Oem one. Have several maybe give it a go in the future.

SoCal250 · February 26, 2020, 12:54:04 AM

Calipers aren't the issue, and they don't make repros for this bike.

Right now I'm just trying to resolve my master issue and get it back on the road with the parts I have. My bike is a RD400F so I'm screwed by the Yamaha engineers who decided it was a good idea to use 1-year-only parts on it -- master cylinder, calipers, rotors, etc.

Next I'll be looking into upgrading my master to a better (modern) one, but I don't want to get into a huge amount of fab work. I could have easily paid for a nice Brembo unit with all the rebuild kits I've been through recently...not to mention the huge amount of my time that has been wasted chasing issues.

ace787 · February 26, 2020, 07:08:54 PM

I was gonna try this place, but mines working...so far
http://www.karpspb.com/

Dvsrd · February 26, 2020, 10:09:25 PM

Quote from: SoCal250 on February 26, 2020, 12:54:04 AM
Calipers aren't the issue, and they don't make repros for this bike.

Right now I'm just trying to resolve my master issue and get it back on the road with the parts I have. My bike is a RD400F so I'm screwed by the Yamaha engineers who decided it was a good idea to use 1-year-only parts on it -- master cylinder, calipers, rotors, etc.

Next I'll be looking into upgrading my master to a better (modern) one, but I don't want to get into a huge amount of fab work. I could have easily paid for a nice Brembo unit with all the rebuild kits I've been through recently...not to mention the huge amount of my time that has been wasted chasing issues.

SoCal,
At least when it comes to the discs, they look very similar to XS650 discs, 77 and forward. Maybe the calipers are the same as well?

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RD400 rear brake master