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Rear master cylinder

Billsboat

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Rowlett Texas
Hi all! I have a 1972 Rickman. I’m the second owner. When I got the bike a few months back, both the front and rear brakes did not work. In fact, they were both dry as the bike had sat for about 7 years. I rebuilt the front master, and cleaned the calipers. The front now works as it should. I have also rebuilt the rear master... this is where I need help!

So as mentioned, I rebuilt the master. I’ve bleed the thing I don’t know how many times. I’ve also used the vacuum method, and I still get the same results. It pumps up, but once u release the pedal, the brake fluid refills back into the tube and I have no pedal.

I fairly sure the master cylinder is from a 1978 Honda super sport cb750f.

Thanks for any guidance!
 
You can rebuild until the end of the earth yet if the master bore is not perfect then fluid will simply bypass the piston in a leak all day long. The fluid leaks out through lows which a honing job does not correct unless you remove way too much material, then the seal cup edges roll over backwards to quickly destroy the cup.

Back hydraulics like that have more trouble than front, they are mounted down low to catch water in them all day long and tend to corrode worse than fronts. I had fits with them on the F DOHC bikes, most of the M/C castings are dead, you cannot hone that much damage out of them.
 
Thanks for responding! I had not considered the reason for the issue I’m having with the M/C but it makes sense.

Frankly I was thinking I was gonna need to replace the M/C. I just wanted to make sure there was no more I could do.

Any recommendations for replacement?
 
No, you generally do not have the options you do on the front. The backs are always proprietary to the nth degree. Not saying something else won't work but I for one don't know of it. The DOHC has the same issue, I think they found one only off a CBR that works with slight modding, nothing else except the OEM part.

Maybe Dirtdigger here onsite can offer something up. For sure you're gonna want to strip your cylinder down to look at the bore with a strong light to see how deep the corrosion issue is.
 
the second picture it of the old master cylinder parts.
 

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Good morning. I’m still struggling with my rear brakes. As it turns out they r from a 1976 750f super sport any recommendations for replacement?
 
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