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Discussion Starter · #1 · (Edited)
Hello All,
I recently acquired this 1980 CB400t Hawk, I believe everything is in fine working order other than the front brakes, where the lever is quite hard and are not very powerful, and after about 100kms have started to squeal.

Here's a picture, I will be removing the crash bars as they vibrate and interfere slightly with the cables. I haven't made up my mind about the luggage rack quite yet.

IMG_6654.jpg

Anyway, I've ordered a new master cylinder because when I tried to bleed them the first time, there was not enough pressure being generated to get the fluid out of the bleed nipple. I've since realized this may not be the problem because I clamped the brake line immediately below the master cylinder and the lever became very hard. The brake lines look like they've been replaced as the bolts at either end are pristine.

I noticed when trying to reverse bleed (with syringe applying pressure from the bleed nipple so that the fluid is pushed out through the master cylinder), that brake fluid leaks from the bleed nipple (only when opened, no leaks when closed). Is it possible that the bolt is stripped and now doesn't seal properly?
After using paper towels to absorb the leaking from the bleed nipple, I managed to get some amounts of quite rusty brake fluid to come out the top. Now seems like the lever has become a bit softer, but the brakes go from weak at first to grabbing with a big squeak as they stop (compared to a metal on metal squeal before)

There's a very small space (just enough to see light) between the brake pads and rotor without brake, but you can see them grab and release upon brake application/release.

I'm thinking calipers are what I should take apart next to investigate condition of piston etc. but I'm not really sure how I can drain the system before doing this with the leaky bleed nipple and the lack of master cylinder pressure.

Hopefully some of my tests are useful and can be made sense of by someone
 

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Some would re-build the master cylinder but my personal preference is to replace unless you're going for a restoration. It's helpful if you can find a master cylinder with the reservoir mounted at a similar angle to the original. Good idea to disassemble and clean the caliper but it sounds like it's working now. The brake pads on a disk brake don't actually "retract", they just release pressure.

Welcome to the group.
 

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Discussion Starter · #4 ·
Some would re-build the master cylinder but my personal preference is to replace unless you're going for a restoration. It's helpful if you can find a master cylinder with the reservoir mounted at a similar angle to the original. Good idea to disassemble and clean the caliper but it sounds like it's working now. The brake pads on a disk brake don't actually "retract", they just release pressure.

Welcome to the group.
I ordered the David Silver Spares one as it looks like it's an exact replacement but was hoping I might have missed something simple so I can get riding it faster :(

Good to know that the brakes aren't supposed to come far off of the rotor, this is my first set of motorcycle brakes so I'm still learning what's normal. I did just check out the brake hose and while it was fine, the master cylinder has brake fluid dripping out so definitely serious issues there.

Thank you for the welcome and I will update if I still have issues after replacing the master cylinder.
 

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The bleeder on the caliper is fine, once loose fluid can go around the threads. It actually only seals against the tapered end. When bleeding if the hole in the center is partially plugged then the fluid will look for another path = threads. Flushing the system out is just a case of bleeding it repeatedly until clean fluid comes out, should be done every couple of years
 

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Discussion Starter · #6 ·
The bleeder on the caliper is fine, once loose fluid can go around the threads. It actually only seals against the tapered end. When bleeding if the hole in the center is partially plugged then the fluid will look for another path = threads. Flushing the system out is just a case of bleeding it repeatedly until clean fluid comes out, should be done every couple of years
Thank you, that makes perfect sense, I noticed there was rusty gunk in the hole when I first took the rubber cap off. Brakes have arrived just have to go pick them up, so I will hopefully get it put together this weekend and provide updates.
 

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Discussion Starter · #7 · (Edited)
Installed the Master Cylinder today, in my opinion it suits the bike better than the stock one.
IMG_6671.jpg

I can't push the bike forward with the brake on anymore so I will say it was successful

For anyone curious, it fit without any modification & the brake switch works correctly. Seems like good quality.

Thank you for the help everyone.
 
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