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It all started with the brakes...

80CB750F

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Hey folks,


I signed up to the forum because I need a substantial amount of help.

I have a 1980 CB750F and it has about 25k miles on it. It all started on a ride home from work. I found my brakes to be more and more taunt everytime I stopped. Turns out my pistons we not releasing as they should have and I ended up stopping and having to release the calipers by bleeding the pressure to get home.

I tried bleeding the brakes and then the master cylinder stopped working. Upon closer inspection I found a SUBSTANTIAL amount of grit grime and other crap that clogged up the whole works. I ordered a new master cylinder kit as well as a new reservoir. I also cleaned both calipers and reattached them. I attempted to refill the lines with fluid and nothing was working. So I worked from the calipers up to find that even when I took the Banjo Bolt off the M/C assembly I could not get Fluid to pump. I cleaned and boiled everything so I thought I was good to go. What would prevent the m/c from pumping up at the top? I notice that in the assembly that there are two drilled holes. One is open and I can see the cylinder moving as I pull the handle. The other one appears as though it was never drilled through to begin with. Are both ports meant to be open? (seems like a stupid question, however that clog looks like metal.) I hope that is the only issue however I feel I would have to drill the hole to clear it and want to be absolutely certain prior to doing so. I decided to purchase a pump to pull the fluid from the bleeders in the calipers if my issue truely is just rouge air bubbles.

On a second note. When I ordered the brake kit items I decided to throw in new drag bars. I went from the stock 4" rise to 0" rise and lost some width from the sides. My clutch cable performed fine, but my throttle was really sticky. I have it in my mind that I need shorter throttle cables but wanted to see if there are any suggestions to avoid that purchase as well.

Thank you!
 
Hey folks,


I signed up to the forum because I need a substantial amount of help.

I have a 1980 CB750F and it has about 25k miles on it. It all started on a ride home from work. I found my brakes to be more and more taunt everytime I stopped. Turns out my pistons we not releasing as they should have and I ended up stopping and having to release the calipers by bleeding the pressure to get home.

I tried bleeding the brakes and then the master cylinder stopped working. Upon closer inspection I found a SUBSTANTIAL amount of grit grime and other crap that clogged up the whole works. I ordered a new master cylinder kit as well as a new reservoir. I also cleaned both calipers and reattached them. I attempted to refill the lines with fluid and nothing was working. So I worked from the calipers up to find that even when I took the Banjo Bolt off the M/C assembly I could not get Fluid to pump. I cleaned and boiled everything so I thought I was good to go. What would prevent the m/c from pumping up at the top? I notice that in the assembly that there are two drilled holes. One is open and I can see the cylinder moving as I pull the handle. The other one appears as though it was never drilled through to begin with. Are both ports meant to be open? (seems like a stupid question, however that clog looks like metal.) I hope that is the only issue however I feel I would have to drill the hole to clear it and want to be absolutely certain prior to doing so. I decided to purchase a pump to pull the fluid from the bleeders in the calipers if my issue truely is just rouge air bubbles.

On a second note. When I ordered the brake kit items I decided to throw in new drag bars. I went from the stock 4" rise to 0" rise and lost some width from the sides. My clutch cable performed fine, but my throttle was really sticky. I have it in my mind that I need shorter throttle cables but wanted to see if there are any suggestions to avoid that purchase as well.

Thank you!

I had the same problem on my '82 750 custom. I couldn't make it home when I purchased it without having to release the pressure in the brake lines. I knew there was an issue when I was purchasing it but I have also seen similar problems before on other bikes I have owned and I knew it was a fairly easy fix. I assumed that since the bike only had 16k miles on it but it was 31 years young it probably had a substantial amount of build-up around the caliper pistons of brake dust and road grime that has hardened up over the years. This gunk and debris was not allowing the pistons to retract enough into the caliper to properly release the brake. I also figured due to its age if was probably time for some new seals while I was in there. Here is my suggestion to fix the problem and the steps I took to correct it.

1. Buy a NEW master cylinder and reservoir, they are around $25 on either ebay or Amazon. They also come with the new banjo bolt (the original reservoir was dis-figured from the sun and looked crappy, I did not want to rebuild it plus I had the piece of mind that the new one was properly assembled and not worn-out.)

2. Get a brake caliper rebuild kit w/ caliper pistons. (someone with the bike in the last 30 years used pliers to remove the pistons and marred them up pretty good. It is essential that the pistons stay clean and free of surface deformities that can collect funk and damage seals. ONLY remove pistons by pumping them out manually with the brake system or you might get lucky with an air compressor if you are feeling brave. Clean out the calipers good with brake cleaner and replace all the piston seals with new ones.) When you are re-assembled be sure to "prime" the calipers through the bleeder best as possible before you start bleeding. The pistons were the most expensive part of this project for me $40 per set and I needed 2 sets. If your pistons are still smooth be sure to clean them up really good and you should be okay.

***you are smart to have a pump to pump through the bleeder, be sure to have plenty of fluid and lots of brake cleaner don't be afraid to use it liberally on any surface to remove fluids, grease or oil from your bike both now and in the future. It is truly a bike mechanics best friend.***

I spent just under $100 for the seals and pistons for the calipers and under $30 for the master cylinder. Don't skimp on buying new brake parts they just might keep you alive. if you haven't already get or save for some steel brake lines, they probably will outlast the bike and give you great piece of mind ;)

Good luck and All the Best!
 
It's Done

Fixed it!

After replacing the cylinder there were issues getting the system to bleed. I bought a pump and still could not get the fluid to fill the system. After a short discussion it was decided the issue was due to one of two machined holes in the assembly that feed fluid to the cylinder. It appeared the one that was closer to the exit (pictured below) was blocked or not fully drilled (not sure how that could ever happen). But we drilled through it and instant success!

Also got new drag bars put on and she runs great!

I'll have a picture up tomorrow.

Thank you for your advice.
 

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Does anyone replace the 30 plus year old brake lines?
I like to be able to stop and stop fast if I need to. I do not trust brake lines from 1980 that have sat with DOT 3 in them for all of those years no matter how clean the fluid, pistons, masters now are. I just got my CB and haven't made it to the brakes yet but I am not riding it either.

Shot I see nice galfers for like $100.00 on flea bay. Any thing less expensive is a SS braided line?
 
Bleeding properly it the trick

Bleed like a car, farthest first.

On my 78 CB750K, just did the same. Lucky for me I have a "Clymer" repair manual for the 1969-1978.

After filling (by hand) your brake "hose" with fluid, so it flows freely out of the caliper bleeding bolt at bottom, tricky to do but you have to keep pressure on the master cyl. exit port, with one finger, pump the master brake lever to do so, and attach the threaded hose bolt with the speed of a flying bullet and NOT ALLOW TOO MUCH FLUID PRESSURE to escape in the process.

In essence, "THERE ARE TWO BLEEDERS" (younger mechanics can't "grasp" that idea, been there, done it, saw it, proved it)

DO NOT DRILL ANY HOLES OR SCRATCH THE PISTON SLIDE IN THE MASTER CYL. (where you said you saw two holes)

your hose should also have a NEW "brass" washer, not smashed, or grooved (potential for sucking/getting air in your hose)


just2cents

good luck, be safe out there.

Lar
 
just posted, just learning to look further, in essence read more and speak less. my bad. glad you got results and thanks for teaching me something new for today... cobwebs are hard to clear some days. :laugh: Lar
 
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