Topic: CB750K4 and the front brake saga!

[andy750]

Hi all,
Today myself and Chris tried for about 7hrs to get my new front brake pads installed on my very stock CB750K4. Should be easy right? Noooo! Heres why...

First off, I installed a new brake piston and rubber seal on caliper A (after a thorough cleaning) and then the new brake pads. Tried this and the brakes worked but wouldnt release....wheel stuck.

Ok so took caliper off again and pressed piston back in. We thought perhaps the brake pad itself was not fitting properly as it took some force to "pop" in. We compared with a brake pad from my K2 and hey this popped in easily! (NOTE: I bought 2 sets of brake pads from OLD BIKE BARN and both aftermarket sets did not install without some forceful pushing in). Ok so front brake pad was sticking - this must have been it.....after re-assembly with worn K2 pad, same problem...piston coming out but not retracting.

We checked the rotor, front brake hinge and both seemed straight so not this. Caliper B was switched with a known caliper B that worked and this didnt solve the problem. We changed the master cylinder and this didnt solve anything. We changed the bleed nipple and this didnt solve anything. We changed the metal line that goes into caliper A and this didnt solve anything. We changed caliper A with another caliper A (and different piston and seal that were ok) and this didnt change the problem.

So whats left? The brake lines from master cylinder to brake switch and line from switch to metal line. Could this really be the cause of non-retracting piston?? We can pump brake fluiid out ok (as in bleeding the brakes) but is there enough pressure to move the piston back in?

Any thoughts or has this happened to anyone else and what did you do to solve?

Frustrated and brakeless,
Andy in Boston

[andy750]

Since I bought 2 sets of brake pads from OLD BIKE BARN that didnt fit properly has anyone bought them from their that did work properly?? Just curious.....
Thanks
Andy in Boston

[MikeDeB]

So whats left? The brake lines from master cylinder to brake switch and line from switch to metal line. Could this really be the cause of non-retracting piston?? We can pump brake fluiid out ok (as in bleeding the brakes) but is there enough pressure to move the piston back in?

Any thoughts or has this happened to anyone else and what did you do to solve?

Frustrated and brakeless,
Andy in Boston

Ok.  How's the caliper hanger?  Does it freely move side to side when there's no caliper mounted.  Off the bike it should freely move all the way side to side with the fork hanger stopping it.  If it doesn't move at all or just moves a little and stops then it needs to be overhauled.  Limited or no movement in the hanger usually means the pin has siezed to the caliper arm and is only rotating in the bracket mounted to the fork leg and it's movement is limited by the cotter pin.  This may be the cause.  If you take it apart there are two o-rings that fit in recesses in the caliper mounting arm; don't lose them.  You will also see there is some relief in the center of the pin.  This is to hold grease.  Lithium grease works quite well.

The second cause of the front brake dragging (and I assume this is what's happening) is the smaller of the two holes in the master cylinder being clogged with crud.  A strand of wire from a wire brush will clear the hole.  The smaller of the two holes is the one that allows pressure relief in the master cylinder when you release the lever.

I've found the above two things need to be spot on in order for the brake system to work properly.  You will still get a little rubbing of the pad (the one on the piston side of the caliper) on the rotor in a properly functioning brake system.  That one doesn't completely retract away from the rotor.  Only the non piston side pad retracts away from the rotor.

Hope this helps.

[mick750F]

   My only experience with non-retracting calipers was caused by a failed master cylinder. When this would happen I could get the caliper to release by opening the bleeder. Are you sure that both mc's are in good shape?

Mike

[andy750]

Thanks guys for the ideas. The caliper hanger was checked and that was in good order. Ill try the wire down the small hole in the mastercylinder as I havent tried that yet so thanks for that suggestion. It could well be that both mastercylinders are bad so Ill double check this with the wire method.

Cheers
Andy in Boston

[Glenn Stauffer]

Since I bought 2 sets of brake pads from OLD BIKE BARN that didnt fit properly has anyone bought them from their that did work properly?? Just curious.....

Almost all the brake pads I've bought have not fit properly.  Most of the time, the paint on the pad carrier is too thick.  I just take a file or sandpaper to the pad carrier until the pads fit into the caliper bore without binding.

--Glenn

[jaknight]

Hey Andy, I have a 750 K4 ~~ everytime I change the brake fluid to cut down on the amount of water being absorbed by that fluid, which it is great at doing and pitting all the aluminum surrounding it, I make a major effort to make sure that the little hole in the master cylinder is clear and clean.  It is nearly always the trouble spot when doing your front brake.  Also, even when it is clear, on my bike I always have to go for near absolute maximum adjustment on the hex nut and bolt for the caliper return spring.  I do hope this will help you ~~ jaknight