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73 CB750 front brake issues

mnvkngs65

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I just wanna ride!!!

Still with front brake issues... I rebuilt the master cylinder and it appears to function with check valve working. I also rebuilt the brakes including new piston seals. The brake lines were not replaced but were cleaned out with wire and compressed air passes easily.

Everything was reassembled and the brakes were bled without problem. The issue now is the pistons will not retract. I verified the check valve in the MC is open (brake fluid will easily and clearly "back" flow from the check valve hole when I apply pressure to the pistons).

The pads are not interfering with the movement of the pistons and the pistons are spotless. When I removed the brake to exam the pistons, check valve patency, etc. I noticed it takes a lot of manual pressure just to push the piston back into the brake housing, way more than I would expect.

I'm assuming it is a pressure issue somewhere along the line but all the lines appear open. I am going to try and to push the pad back in again after opening the bleeder valve. If it pushes in easily I will assume again it is a pressure problem not an issue with the piston, seal or brake housing.

Any thoughts?

Thanks!
 
If you have the separate little check valve that goes in the very end of the M/C and a separate part from the M/C piston itself, it may be that. My 550F showed same part as needed in the assembly on the parts fiche and I removed it to have the M/C work perfectly for years. The piston itself is a check valve if it has both a one way seal and the 6 tab reed valve riveted into the end. And if you added any washer or such behind the circlip to shove the piston up slightly forward to not be able to access BOTH the larger hole in bottom of cylinder reservoir as well as the one with the pinhole size opening, the brake will NOT let off 100%. That pinhole is a must, it lets pressure fall off with use to let brake go back to zero pressure. Why it is called the compensation port, it allows the pressure to equal at brake off. The edge of piston seal MUST come back just barely past that last very small hole.

The screw that sets the bracket that holds caliper to allow it to slightly swing with brake pad wear must be properly adjusted or brake drags too.

I always slide the piston back and forth in the caliper housing checking for 100% loose before putting the piston seal on it, once on you have no idea if the piston is dragging on bore or not because seal drag covers it up. That early design needs a minute amount of grease smeared on the piston OD outboard of the seal (crazy, I know!), it allows the piston to slide, not having it the caliper aluminum will corrode in only a few days to lock the piston and stop it from backing off. Because the early design had not incorporated a dust seal there yet. So if yours like that there's that issue.
 
Thanks for the info. Still lost. Everything appears functional. Compensation port is open with fluid able to be expressed easily. Pistons in the brake caliper are spotless and able to slide past the seal prior to adding brake fluid. When I opened the bleeder valve last night I was able to manually push the pad/piston back into the caliper fairly easily.

I hate to break the whole system down again but....
 
You MUST have TWO ports in master floor feeding the piston, having only one is not enough. You do not say that.

A few of the M/C pistons change configuration too, you must be sure of the correct one being used. If M/C is working right you do not need the extra check valve Honda added that goes in the master bore close to the end for hose and I do not understand why Honda used them, unless the piston is missing the reed check on the end of it.

Not clear either on what you mean by caliper 'pistons' (as in plural), they are single piston only unless running dual discs. One piston only per caliper.

FYI, pistons never truly retract, they only go outwards. The way the brake backs off is that the piston strains forward enough to warp the seal toward outward before piston moves, when you let off the brake the seal then goes back to original flatter shape and the warping back in is what releases the brake. You only move maybe .010"-.020" max to do that, the piston should not 'back up' other than to let that seal relax. That action of the piston constantly self-advancing in very small steps is how they stay automatically adjusted.
 
Thanks for the info.

Sorry, didn't mention the brake configuration was changed to 2 discs at some point prior to purchase and I assume were working at some point as well.

Both ports of the MC are open and functional (brake fluid can be expressed from the smaller port when pressure is applied to the piston).

I recleaned the calipers and pistons again. This time I reduced the system to one caliper and it seems to be working well. I didn't use any silicone grease on the piston seal this time (I used a small amount previously). I don't know if this was what made a difference or not?!

Would small pitting in the piston prevent the brake from functioning? See attached pic.

I plan to test the other caliper and piston set this week. If functional I will try a 2 caliper system again.

Thanks again!
 
IMG_20190929_124249806.jpgIMG_20190929_124255188.jpg
 
The pitting will leak if deep enough the seal cannot squeeze up into it to seal, the pitting also shreds the seal to leak by itself. Pistons need to be perfect.

If the M/C was not changed to a 5/8" diameter one it is too small, the single disk one is 9/16", you need more volume to work two disks. People do it with the smaller but more problems can show.

You do NOT use silicone grease ON the piston seal, that stops the seal from warping out like needed, it would then slip more on piston groove instead and add to your brake sticking issue. You lube the outward end of piston BEYOND the seal, to stop corrosion buildup between the piston end and the aluminum of the caliper. The corrosion otherwise gets thick enough to lock up the piston. Honda messed up not providing for exposure to water there, they needed separate water seals like the later ones use.
 
Tried the other piston and caliper tonight, didn't work. I'll try a new piston in the caliper to see if that works. If it doesn't, I'll stick with the one caliper system. The MC is off a GL1000.

Thanks!
 
Quick question. Is there anything I can use on the threads of the brake line at the caliper to stop a very small leak or do I have to buy a new caliper?
 
Its a flared fitting so unless there is a nick on the flare in the caliper or the line make sure it is tight enough should be no need for anything to help make it seal.
 
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