Topic: Rear Caliper Rebuild '75 CB750F

[YoungBlood]

Rebuilt the caliper unit: Breaker Crafters (2 new pistons, 2 seals, Copper Washers), also used separate OEM seals. Also bled the brakes and refilled with Dot 4, and installed 2 new EBC pads. Problem - After about 1 miles of riding, the brake pads grip the rotor hard, and just about seize against the rotor. I pulled off the pad cover, pulled pins, pulled clip, loosened caliper set bolts so I could pull the pads... no problem, caliper is not the issue, they are retracting fine. This same thing has occurred again, this time I thought maybe it's the pads are not beveled down enough, so I used a file and filed down the paint, the corners, and the back. Then I tried messing with the pedal, and seeing if somehow the positioning of the pedal is the problem. I don't believe so. Any ideas? Do I need to rebleed these brakes because there is still air in the lines? Could it be that because the caliper set bolts I forgot to tighten until after I bled the brakes that it allowed air into the housing?
 

[YoungBlood]

More pics that were unattachable

[YoungBlood]

One more

[BenelliSEI]

If your pads keep pressure on the disc there are a few causes. Did you rebuild the master cylinder? Change anything in the linkage?
1. Inside the master cylinder reservoir (assuming stock), there are two holes at the bottom. The large one is for feeding the cylinder. The TINY one is for releasing line pressure. Is it blocked with crud?
2. I have seen aftermarket pads that are just too thick. If they slid in easily to start with, then NOT the problem.
3. When you rebuilt the caliper, did you get the grove behind the seals SPOTLESS? I use a small dental pick with a 90 degree bend in the end to scrape them out. After that, a small brass wire wheel brush on my Dremel, to polish them. Any crap in that groove causes the seal to press on the piston and hold it in place. Your brakes are dragging because something is keeping those pistons from going back......
4. Unusual, but I have seen this more than once. If the flexible rubber hose that connects the master to the calipers is original, it may be deteriorating internally. Pedal pressure will force fluid into the calipers, but that hose is so plugged up with it’s own bits that it’s acting as a check valve

Lots to consider..... good luck.

[Jerry Rxman Griffin aka MuthaF'er]

Am I missing seeing the 2 pins?

[YoungBlood]

Thanks Benelli, I'm going to keep these all in mind. I'll check the master cylinder for blockage in the tiny port. Would it make sense to just re-bleed first because I could still have air in the line causing the problem? Also, this extra pressure on the breaks was my initial problem a few weeks ago, hence I decided to rebuild the caliper housing.
1. I'll check the small port when I drain the brakes and rebleed.
2. I don't think the pads are the problem, they dropped in right away like you said.
3.  Yes I did try cleaning that sucker up as best I could, I have a sheet metal scribe, or etcher. A metal pick basically. I only used it in the groove, but it's possible it is not spot-less. I also do not have a dremel, and no wire brush adapter for my drill. I only cleaned it up with carb cleaner.
4. I have not touched the master cylinder, and that hose is not cracked... original? I don't know if it's possible for it to be non-cracked and from 75'.

Also, Jerry, I do have the pins installed plus the clip.

[Hoosier Honda]

In the 3rd picture it looks like the fluid cross over hole has rust in it, my guess is what Benelli said about the master cylinder return hole being plugged, or at least restricted. It is a very tiny hole, like sewing needle size so it doesn't take much to plug it. When this happens on your front brakes things go south fast.

[YoungBlood]

Thanks Hoosier, I'm going to drain the brakes again and look into this master cylinder port. The front brakes were rebuilt by the previous owner... I'll be sure to take a look at the front master cylinder with my owns next.
Both Hoosier, and Benelli, what's the best way to unplug that return hole if I find it plugged?

[ekpent]

 A easy way to check if the bleeder hole in the MC is plugged or the hose is bad is to break open the bleeder screw on the caliper when seized and see if it releases pressure. On front brake masters I have used a wire to open the bleeder hole with the handle partially squeezed in. Had to chuck a small piece in a drill once to clear it. Never done an F back one though yet.

[BenelliSEI]

+1 to what’s above. From what you reported, I’d start with the “little hole”.

You may also find that the introduction of fresh brake fluid (and maybe a change in DOT type?) has finished off the seals in your rear master cylinder....... but check that return hole first. That might be all you need.