Author Topic: CB750-Front Brake Dragging. (Read 21970 times)

Magpie · « **on:** January 21, 2011, 04:21:33 pm »

Hi! If my CB750 sits for long while the front brake drags making it really hard to move around. I rebuild the master cylinder but the fluid must be leaking by the seals, is there a check valve in there I screwed up on? The bore of the master looked good.
Thanks,
Cliff.

BeSeeingYou · « **Reply #1 on:** January 21, 2011, 04:28:56 pm »

I would think the problem may be in the caliper more than the MC. I am guessing brake dust and road grime have built up causing the pad to hang up. And there is a check valve in the MC but as long as you put it in correct it would be hard to screw it up. I don't think the check valve was used after 1976. Also make sure the return hole is clear in the MC, the tiny one nearest the outlet port.

seanbarney41 · « **Reply #2 on:** January 21, 2011, 05:21:22 pm »

And this surprises you? Out of the three old Hondas I've owned, the only one that didn't drag had a drum in front!

Gordon · « **Reply #3 on:** January 21, 2011, 05:23:22 pm »

In the caliper, the groove that the piston seal sits in probably needs to be cleaned out. Gunk builds up which prevents the seal from seating fully, which then prevents the piston from retracting fully.

Magpie · « **Reply #4 on:** January 21, 2011, 05:44:36 pm »

I'm sorry, I should have mentioned this bike has less than 10 miles since the rebuild/restore, driven on clean dry roads. The caliper and piston are clean. Maybe the seal on the caliper is dry and hanging up the piston.
Surprised? Yes, because the one beside it never drags the front brake.
Thanks,
Cliff.

Johnie · « **Reply #5 on:** January 21, 2011, 06:27:05 pm »

Just to get you over the hump if it happens again, take a rubber mallet and give the outside of the caliper a smack and it will free up so you can move the bike around.

HondaMan · « **Reply #6 on:** January 21, 2011, 08:00:00 pm »

Quote from: Magpie on January 21, 2011, 05:44:36 pm

I'm sorry, I should have mentioned this bike has less than 10 miles since the rebuild/restore, driven on clean dry roads. The caliper and piston are clean. Maybe the seal on the caliper is dry and hanging up the piston.
Surprised? Yes, because the one beside it never drags the front brake.
Thanks,
Cliff.

Cliff:
Did you also replace the disc's pads during the rebuild? Reason I ask: the ones I've seen recently (from everyone but Fedoro) have thick paint (usually black) on their edges. This greatly reduces the clearance between the formerly plated (not painted) edges of the disc and the caliper itself. Honda used to zinc plate them, which is a natural lubricant under pressure. The paint also absorbs moisture enough to get sticky, in my observation. This makes them stick in the caliper when not used, then they won't "back away' when the wheel is rolled.

I've fixed the last 4 bikes (here), maybe 5, that were this way by just removing the paint with some light grinding/sanding, then applied some light touch of brake grease around the metal edges. It seems to work out fine, even after sitting all winter while waiting for the owner to come back for his bike....

all of these bikes had the [noisy] EBC pads, with their thick paint on the edges. As part of the fix I was doing, I was adding the ASK-1 material to the back of the pads, too. This lets it have some slight resilience, rather like the nylon rings found in the post-1976 bikes' calipers. This also helps.

Magpie · « **Reply #7 on:** January 21, 2011, 08:22:59 pm »

Thanks guys! I'll bump it with a mallet in the morning and have a look at the edges of the pads this week. Jeez I love this place!
Cheers, Cliff.

Kevin400F · « **Reply #8 on:** January 22, 2011, 04:20:16 am »

If you crack open the bleed screw on the caliper and get a spurt of fluid along with elimination of the drag, then the problem is most likely the master cylinder. There are two holes in the floor of the master cylinder reservior. The one closest to outlet (hose end) is very tiny and often gets plugged, preventing residual pressure from the caliper and hose from "bleeding back" into the master cylinder when the brake is released.

Kevin

ekpent · « **Reply #9 on:** January 22, 2011, 05:23:44 am »

Cliff,Knowing that your resto's are clean enough to eat off from and you are very thourough in your rebuilds I would go along with Hondamans sound advice. As you have so little mileage and the caliper has sat and been used so little I would think it more of the pads hanging from sitting. Worst thing for an antique clock for instance is not to run the same can be said for some of the components of our machines. PS-Thought I read where you and 754 got together on an early 60's little Honda of some kind

Any pics

Eric

Grabcon · « **Reply #10 on:** January 22, 2011, 06:30:07 am »

The main cause for front disk brake drag on these old bike, assuming the caliper is functioning properly is the lower legs of the front end slightly twisting when you move the bike. This will happen whether you have the front fender in place or not. The front fender provides little strength to prevent the lower legs from twisting.You will notice that when you roll the bike turned one directions it drags and turning and rolling the opposite it will not. A simple cure. Put a fork brace on the lower legs. NOT the top tubes. The axle is the only thing holding the lower legs straight. The top tubes have both the upper and lower tree securing them from the twist.

I have a CB900F with the dual disks, same issue. I put a lower leg fork brace on the drag stopped and the overall handle improved dramatically. If you are really going to ride these old bikes with the small diameter tubes on the front end the lower leg fork brace is a must.

ekpent · « **Reply #11 on:** January 22, 2011, 07:06:11 am »

Good point Grabcon as I too have noticed that issue is more pronounced when the wheel is turned.

Silverback · « **Reply #12 on:** January 22, 2011, 08:18:45 am »

Quote from: Grabcon on January 22, 2011, 06:30:07 am

The main cause for front disk brake drag on these old bike, assuming the caliper is functioning properly is the lower legs of the front end slightly twisting when you move the bike. This will happen whether you have the front fender in place or not. The front fender provides little strength to prevent the lower legs from twisting.You will notice that when you roll the bike turned one directions it drags and turning and rolling the opposite it will not. A simple cure. Put a fork brace on the lower legs. NOT the top tubes. The axle is the only thing holding the lower legs straight. The top tubes have both the upper and lower tree securing them from the twist.

I have a CB900F with the dual disks, same issue. I put a lower leg fork brace on the drag stopped and the overall handle improved dramatically. If you are really going to ride these old bikes with the small diameter tubes on the front end the lower leg fork brace is a must.

I've been fighting the dragging brake issue, too. I will be doing a rebuild soon, but I did notice similar characteristics. Whenever I apply the brakes it seems as though they stick. After coming to a stop, I turn the bars left and right and the brakes are unstuck until I use them again(Seems like a sticky piston). I also notice that during hard riding I get brake squeal when turning into corners. It's a bit unnerving because once I'm into the turn I do not want any front brakes. I'll be fabricating a brace that allows me to use the fender. My 78F has a front brace and the automatic turn in braking is absent. It is a different brake system, though.

Grabcon · « **Reply #13 on:** January 22, 2011, 08:23:53 am »

These types of caliper on these old Honda's do squeal when the forks start to twist.

Magpie · « **Reply #14 on:** January 22, 2011, 08:24:02 am »

I'll have to check again if it drags more when the bars are turned.
Eric, it's a 1964/65 C95 Benly in bad shape but fixable. 754 rescued and stored it for me for a bit. If you get to Kelowna ask 754 for a tour of his shop, it's an amazing place!
Cliff.

ekpent · « **Reply #15 on:** January 22, 2011, 08:29:53 am »

Neat bike Cliff,looks like another project on tap.Your painter is gonna love you on that one,lotsa red to do.I saw your thread on 754's shop also,looks like a real cool place, tons of neat vintage items and collectibles.Take care. Eric

MCRider · « **Reply #16 on:** January 22, 2011, 10:15:50 am »

Quote from: Kevin400F on January 22, 2011, 04:20:16 am

If you crack open the bleed screw on the caliper and get a spurt of fluid along with elimination of the drag, then the problem is most likely the master cylinder. There are two holes in the floor of the master cylinder reservior. The one closest to outlet (hose end) is very tiny and often gets plugged, preventing residual pressure from the caliper and hose from "bleeding back" into the master cylinder when the brake is released.

Kevin

Besides everything else mentioned, cleanliness, paint on the edges of the pucks, turn lft/rt. etc. If it is "really hard" to push, I'm with Kevin that its the return hole being plugged with debris. This can be the tiniest of stuff, maybe dislodged in the cleaning process. All the other things are nuisances but don't make it "really hard" to push.

All the other things will produce drag, but "really hard to push" is the return hole IMO. If you smack it with a mallet and it doesn't free up, that means the fluid is not returning into the MC. If as Kevin says you release some pressure at one of the banjo bolts, or the bleeder valve, and it squirts at you, it is storing pressure, return hole.

mrbreeze · « **Reply #17 on:** January 22, 2011, 11:45:32 am »

Quote from: Kevin400F on January 22, 2011, 04:20:16 am

If you crack open the bleed screw on the caliper and get a spurt of fluid along with elimination of the drag, then the problem is most likely the master cylinder. There are two holes in the floor of the master cylinder reservior. The one closest to outlet (hose end) is very tiny and often gets plugged, preventing residual pressure from the caliper and hose from "bleeding back" into the master cylinder when the brake is released.

Kevin

This is where I would go first.It is the easiest of the 2 to check (caliper & master). It was the problem with mine a few years back.If that tiny port is plugged....you can see it free up when you dig around in there with a fine guitar string. Not saying it can't be the caliper but I would check the master first.

Magpie · « **Reply #18 on:** January 26, 2011, 10:26:00 pm »

Thanks guys! I did tap the caliper with a mallet but it didn't help, will try to clear the return hole and see what happens. Cheers, Cliff.

MCRider · « **Reply #19 on:** January 27, 2011, 05:28:27 am »

Quote from: Magpie on January 26, 2011, 10:26:00 pm

Thanks guys! I did tap the caliper with a mallet but it didn't help, will try to clear the return hole and see what happens. Cheers, Cliff.

Did you release some pressure from the bleeder valve or banjo bolt? That will tell you its the MC return hole.

Magpie · « **Reply #20 on:** January 27, 2011, 06:39:33 am »

I did at the bleeder and a small amount of fluid came out. I tried to get more with a Mitivac but the tubing I was using didn't fit the bleeder tightly enough. Cliff.

MCRider · « **Reply #21 on:** January 27, 2011, 09:10:13 am »

Quote from: Magpie on January 27, 2011, 06:39:33 am

I did at the bleeder and a small amount of fluid came out. I tried to get more with a Mitivac but the tubing I was using didn't fit the bleeder tightly enough. Cliff.

So once you released some fluid, did the brake free up so you could push it easily?

Magpie · « **Reply #22 on:** January 27, 2011, 09:35:07 am »

No, it didn't. Sorry, I should have mentioned that in my earlier post. Cliff.

MCRider · « **Reply #23 on:** January 27, 2011, 11:28:51 am »

Quote from: Magpie on January 27, 2011, 09:35:07 am

No, it didn't. Sorry, I should have mentioned that in my earlier post. Cliff.

Wow, not what i wanted to hear.

That would re-indicate something wrong at the caliper. No suggestions from me. Sorry.

Still could be just the return hole, lets hope.

Magpie · « **Reply #24 on:** January 27, 2011, 07:36:26 pm »

Thanks for your interest and help MC. I haven't had a chance to get to it but will tomorrow. Cliff.

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Author Topic: CB750-Front Brake Dragging. (Read 21970 times)