Topic: front brake diagnosing

[Don R]

 I have dual calipers on my K2, they are the center bleeder F1/K7/A type installed with new aftermarket seals, individual SS hoses and fluid last year.
  The issue is, both sides are dragging. I noticed the bike is hard to push until I bump both calipers, I tried adjusting then removing the screws and springs.
 The return hole in the M/C seems to be open. I'm thinking if I pump the brake and it drags then, bleed the pressure off at the M/C and it rolls easy that would indicate the master cylinder is at fault, right? If that doesn't help then the piston seals must be junk. (they are likely aftermarket)
 

[bryanj]

Don used lots of aftermarket seal and never had a problem, got to ask the obvious
1 are the pivot pins super fre
2 was the seal groove extra clean especially the corners

After that yes if the binding eases when master banjo cracked it suggests master

[M 750K6]

I had a sticky pivot from corrosion, which caused the brakes to drag. Now I remove the pivot, clean it all up with emery cloth and re-grease every other service.

[BenelliSEI]

Don…. Split a caliper and have a look. The last one I had sticking like that needed to be pulled apart. I scraped out the seal groove (full of crust), cleaned the bore (above the seal), cleaned up the pads and put it all back together using the same seal. Problem solved.

[PeWe]

Thick pads, powder painted in need of grinding off some paint?

[Don R]

  All good suggestions and rather than say I already did those things, I'll double check each one because something is clearly acting up and I had my hands on every part so it's on me. Thanks.
  It's funny, I was pushing it and thinking jeez this bike is heavy.

[rotortiller]

I had one carefully rebuilt with alternate piston seal and SS piston but it stuck a tiny bit after a year and sweat fluid ever so slightly, changed to OEM and the problem went away. Just saying.

[MauiK3]

I just thought about stainless pistons and aluminum caliper bores, corrosion, even a tiny amount from the two metals?

[BenelliSEI]

That last one I did was while working on the 1970 cb750K0 (middle one in pic). The caliper was absolutely spotless when I rebuilt it +5-6 years ago. JP brought it back for some minor work, including a dragging front brake. When I opened the caliper there was lots of fuzz around the pad and piston (above the seal), but the real culprit was a bit of crusty deposit under the seal. Cleaned it all out, really scraping out the seal groove, and put it all back together. Even reused the seal and perfect now……

I did open a small packet of BOSCH caliper grease (comes with the pads we use in the race car) and lightly coated the outside of the piston. Hopefully this packs that gap, under the pad, and keeps moisture out?

[jonda500]

um..., isn't it odd to include caliper assembly grease with pad sets? Or was it anti-squeal grease?
John

[BenelliSEI]

John….. it’s used on the “slides”. Many cars today only have pistons on one side of the caliper (a bit like our Hondas!). They are more like a huge claw. They need to slide on the “fixed mount” , usually in notched tracks. Those tracks need to be clean and lubricated. These packages are small and the grease seems to stick well and migrate very little. We race twin 8 hour enduros each weekend, using one set of tires and pads a day. I have lots of extra packets!

[MauiK3]

Nice fleet of beautiful red bikes, nice shop!

[bryanj]

Sorry, i just grab the coppaslip, use it on damn near everything

[BenelliSEI]

Sorry, i just grab the coppaslip, use it on damn near everything

Bryan…. Me too, that’s my “go to”,  but I have a few dozen of these and need to use them up!

[bryanj]

That should be HT so good for it

[jonda500]

so the piston can be lubricated with any high temperature grease?? I have always used just brake fluid as I thought you would have to use special brake assembly grease that's probably expensive!
   My Datsun 720 has a weird caliper set up that's on one side of the disc, but with two pistons in one cylinder forced apart by the brake fluid. One piston pushes one of the pads directly onto the disc, the other piston pushes a claw "G" bracket which pulls the other pad onto the disc! It may be possible to convert a CB caliper to work like this with a lot of machining and fabrication, but would the braking force be improved? I have puzzled over this long and hard but can't see that this would actually give any increased leverage?
John

[BenelliSEI]

John…. I also smear a small amount of brake fluid on the seal so the piston slides in easily. I only smeared a bit of this “brake lubricant” around the top lip of the piston in an attempt to fill the gap illustrated in my poor sketch. That seems to be where most of the corrosion collects that leads to “sticky” brakes. No idea if it will help, but certainly not harmful…..

[BenelliSEI]

Sketch

[Don R]

 I dug through parts while cleaning the shop, I had one F seal and one piston. Have not done any recon on the bike though. The V65 calipers on my 78 gl have an outside dust seal to keep the piston groove clean.
 I used to put a little silicone grease on the piston to get it slid past the seal and was told not to do that, just use brake fluid. I always wondered if adding round o ring outside the piston seal area would help with keeping dirt out but decided why re-invent the wheel?

[BenelliSEI]

Don…. I looked through my “0” rings when I did the last one, having that same thought. I couldn’t find one with a narrow enough cross section, so used the bit of lubricant instead. I’ve used this on the last few I did. It will be interesting to see if it makes any different.

[Kelly E]

We use a really thin wipe of red brake assembly grease. It's what they use to assemble new/rebuilt calipers so they don't dribble brake fluid during storage. We get little tubs of it every time we buy rebuild kits from brakecrafters.com and it's more than you need for one job so we have a few in stock.
We have not had any issues but I can imagine problems if you just slobbered it on everything.

[HondaMan]

Don't forget to check the brake lever's 'soft stop' in the master cylinder. Honda had the brake lever sit partially "pulled" a tiny bit, first by using 4 expensive pieces (a sheet metal plate, fancy screw-offset cam, locknut and flat washer) as the "lever stop" in the K0/1 (and some of the early K2) CB750s. The setup for the fancy one was usually done wrong by (American) techs, who would set the cam for no-contact with the lever, probably because Honda didn't make clear the reason for this stuff. Someone [probably accidentally] discovered that installing a short piece of 3.5mm vent hose would hold the lever in almost the exact-right position, and in late K1 bikes (New Factory production) the inner parts vanished and a sheet metal cover held a 3/8" long piece of rubber hose in the hole, instead. This worked well enough that all production changed to that through the F1 bikes.

But...when the hose gets old, it is no longer squishy, and then the lever doesn't get held in the partially-vented position. When this happens, the brake pads can slowly creep outward as temperature changes down-up-down-up nudges the puck(s) out a little, making it "set the brake" as we called it when pulling bikes out of winter storage. The 750K2, first to get all-rubber-hosed master cylinders, started showing this tendency over a single winter as early as 1974.

The fix: cut off a new length of 3.5mm hose and replace it in the master. I've fixed quite a few of these on bikes that sat for years, then came to me for rebirthing.

[MauiK3]

Wow, great information. The production history of these bikes is very interesting.

[newday777]

Wow, great information. The production history of these bikes is very interesting.

Honda was constantly changing parts as the need arose, so that why as parts manager we would ask for the vin numbers to get the correct parts.

[PeWe]

I made a mistake once when I lubed piston/rubber seal with silicone grease.
I misread the manual. It should be outside on rear side of brake pad. Anti squeal. Noice reduction.

I got a huge brake fluid leak rather quickly during the first ride.

After that only brake fluid on caliper piston and its seal.

My car has floating brake calipers. Lithium grease is a must on 2 pins so caliper can slide sideways , both pads to get the same bite and wear both sides of the rotor evenly.