Topic: CB350F Front Brake Dragging, Please Help

[nrowah]

Hi SOHC4 Forums,

I finally changed my tires but upon putting my front wheel back on I found that the front brake drags. I've been trying to fix it for a day now, but I have had no luck. Anybody have any advice?

Some details:
-I cleaned and cleaned/lubed pivot
-Removed/reinstalled piston and seal (seal had decent edges, but piston had some pitting)
-When pushing the piston back, I noticed fluid coming out of the return hole in the MC

Any advice for what I can do next? I've heard that the return hole clogs often, but Im not sure if mine is clogged. Also I heard that if a piston is pitted I should replace it, but will it actually make a difference? Can the forks out of alignment cause a similar problem?

Thanks,
Nathan

[scottly]

If the brake wasn't dragging before replacing the tire, there may have been an error in the reassembly; was the caliper removed? If so, a common error is to fit the fender brace between the upper caliper mount and the fork leg. Another common error is to reverse the axle: the nut end of the axle should be on the left side.

[Trevor from Warragul]

Have you adjusted the pivot bolt so the caliper is centered on the disc?



e.g. the bolt with the spring

[gtmdriver]

As morini says these calipers are adjustable.
Make sure you've adjusted it correctly after stripping and re-fitting.

[nrowah]

If the brake wasn't dragging before replacing the tire, there may have been an error in the reassembly; was the caliper removed? If so, a common error is to fit the fender brace between the upper caliper mount and the fork leg. Another common error is to reverse the axle: the nut end of the axle should be on the left side.

I don't remember it dragging before, but the only thing I changed when I put the wheel back on is that I accidentally pumped the brakes once or twice while the wheel was off. I pushed the piston back in though. Looks like the lip part of the axle is on the left side, so I think it's on correctly. Also the parts all look like theyre on again in the right order. Fender brace-->caliper swing arm mount-->fork leg

Have you adjusted the pivot bolt so the caliper is centered on the disc?

e.g. the bolt with the spring

I took off the whole assembly, cleaned and lubed the pin. Put it back on and adjusted so that pad B made contact, then backed off 1/2 turn so there was a tiny bit of space. Wheel spun free until I pumped brakes once, then the dragging starts.

Could it possibly be the piston and seal? There was some pitting around the exposed part of the piston, and the seal looked like it had seen better days.

[jaytee-nz]

The piston is supposed to retract slightly when the pressure is released and the seal helps with the retraction.
Looks like you need a new seal and if the piston is pitted, a new piston as well.

[Bodi]

Is the brake lever feel as firm as before all this work? Air in the system tends to defeat the piston pullback as well as giving a mushy feel at the lever.

[robvangulik]

Did you clean the seal groove thoroughly?

[Deltarider]

Not much help from me, I'm afraid, Nathan, but does any of you have a clue why the CB350F is particularly prone to this front brake dragging? Back then I've heard more than one horror stories of the front brake locking en route. It always concerned a CB350F.

[robvangulik]

Probably due to not being used of the bike, as the caliper is identical to that of the 400 / 500 etcetera, and i have never had the problem with any of the many sohc's i've had over more than 40 years.
Good maintenance and regular use is the best life for every bike!

[BazF]

Just got a 72 350f that would not move, front brake locked up. I had to release the caliper mounting bolts to move the bike.
After trying adjustments for a while, I took the brake pads out, removed the plastic spacer behind the left pad and no more problems, good easy adjustment, no binding, good brakes. I think the PO must have fitted that spacer, not shown on parts list. Take a look at yours.

Baz

[TwoTired]

With the caliper piston pushed in, the stationary pad is adjusted for .006 in. clearance from the disk.

After the piston is activated, it is the job of the square section seal to pull the piston away from the disk.

If there is excessive piston pitting, the square section seal distorts irregularly and can't pull the piston back properly.

Also, what did you use to lube the seal?  Brake fluid or brake assembly lube (later much preferred) is ok.  Silicone or petroleum lube is not.  The seal has to be able to grab the piston in order to pull it back.

Cheers,

[mwvachon]

What kinda shape is the master cylinder in? Is the brake fluid the color of tea? Consider rebuilding this if not done so already. A plugged relief orifice will prevent the piston from relaxing properly, creating the type of drag you've described. This is assuming you've already tended to the caliper piston & seal already...

[754]

I bet cleaning the seal  ring groove helps ..

[Deltarider]

I bet cleaning the seal  ring groove helps ..

+1.
BTW, if you have that rubber ring out, you will see the ring has deformed such that one side is a bit higher. You can use this. Install the ring so that the high side of the ring is an extra help to retract the piston. Apply some Ate or similar paste that is compatible with brake fluids.

[strynboen]

yes that ATE stuff is great..have used it for 25 years..on all kind of brake systems..from Volvo 200 serie to sohc and cbx..think it good up to dot 4 or 5..have no idea if it kompitabelt to modern fluids....

[nrowah]

I replaced the piston/seal and it kind of worked. At first it wasn't dragging at all anymore, but the piston was able to retract too far I think (the lever needed to squeeze farther), so I adjusted the pivot bolt to bring the pads closer together (tightened until pad B contact then backed off 1/4 turn). That helped the lever feel a little but now the brakes drag again, but less this time around.

Small detail: I accidentally twisted the seal, but it's a square cross section so the orientation shouldnt matter, right? When I installed the piston it slid all the way in on its own without any pushing. I havent seen any brake fluid leak past so Im assuming it's sealed up fine, but Im not entirely sure.

I did bleed the brakes until the bubbles stopped coming out and then some, but the lever feel still isnt as firm as it was before and the brakes still drag a tiny bit. I guess I can try taking it apart again. Any suggestions for round 2?

[strynboen]

if you can push the piston inn ,vith 2 fingers..all ok there.(need to be a bit back pressure)if it falls in..there is a problem vith the sqvare o ring,..

[jaytee-nz]

Every new seal I have put in required the piston to be pushed in with a bit of force behind it. I can't remember specifically but have a vague recollection that the seal wasn't square but oblong. If that is correct then putting it in on the short side as a result of it twisting will mean the ID is larger which would explain the piston slipping in by itself. I would think that will eventually lead to leakage.

[Deltarider]

the lever feel still isnt as firm as it was before and the brakes still drag a tiny bit.

I'd leave the bike standing for a night with the brake lever pulled (use some strong elastic bands). Not fully but say halfway, so eventual airbubbles can find their way up. And... are you aware that a disk brake is supposed to drag a tiny bit?
Still I wonder why the CB350F had this locked brake syndrom more often than the other models. If the parts are more or less the same, could it be the CB350F's brake was more exposed to dirt, due to a smaller front wheel/tyre?

[strynboen]

most hardvare is the same on 350 400 500 550..only the disc,is sleightly smaller in diameter and vheel is 1" smaller..think most 350 have been stored long time..and new  ovners(/builders..do not take the time to split it,,and klean aal up..before try to running the bike...its a beginner bike for most builders..after ve have made some bikes..ve learn that it need to be split and kleande up..to funktion perfekt..you have to learn the prioerty. of repaieres.brakes is first ..engine tires elektric kaburators.and on and on...

[bryanj]

You need to have the seal groove in the caliper half scrupulously clean as it is the fractional twist in the seal rubber thar pulls the piston back and the crud in the groove stops that happening