Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: ColinHH on August 15, 2024, 09:12:43 AM
-
Sorry, but you can probably tell that I am doing quite a bit of work on my K4 at the moment and thanks to this group I am gradually getting through it. Electrics done, front fork oil seals done and now looking at the front brake caliper.
My first question is, how freely should the front wheel spin when brake not being applied? With the pads depressed and the back pad adjusted with the swing arm screw, so that it is just off the rotor, the wheel free wheels and will turn 3 or 4 revolutions before it stops. However I then pull the brake lever and obviously the brakes engage and the wheel stops really well, but once I release the brake lever the wheel won’t free wheel and whilst I can turn it the pads are binding. I think this is why it has been so difficult to move it around in the garage. I took the caliper apart and there is some pitting on the piston! I cleaned everything and very lightly used some wire wool to clean around the metal edges of the pads. It still binds. So is this normal or do I need to get a new piston and seal or is there anything else that I should be looking at? Thanks.
-
This is normally caused by the moving pad not retracting,
Can be caused by it being a tight fit in the caliper, or,as well the piston seal is old,gone hard so not pulling the piston back,
Have you replaced the seal?
Sent from my SM-A546E using Tapatalk
-
agreed John. I cannot remember how many times I have rebuilt my front caliper. Periodically, it starts to stick and the only remedy is to strip, replace seals and often the piston as well. Currently have a stainless steel piston in and fingers crossed it seems better. Luckily, it is very easy to rebuild these and the brake bleeds very easily.
-
Thanks, new seal and piston just ordered. Another thing worth mentioning is that the “static†pad (The one that is held by the clip on the inside of the disc) is virtually worn down to nothing but the moving pad (the one which gets pushed by the piston) is hardly worn at all!
-
That might be due to the top pivot being seized as after the brake has been applied the fixed pad needs to be pushed away from the disc by the natural high point as the wheel rotates. Any binding on the pivot action will wear the fixed pad away.
-
Haha! I just read the response regarding the pivot arm just after I got back from the garage looking at exactly that. The arm wasn’t pushing the fixed pad back. So I used some WD40 on the pivot and put a small spacer between the spring and swing arm. Problem solved! I have already ordered the new piston, seal and pads so won’t do any harm to replace them anyway.
-
Ideally you need to remove the arm from the pivot and use Copper slip or something similar as the WD40 will only be a short term fix. A decent lubricant that stays in place is needed imho.
-
Thanks Ted. Will do as you suggested when I take it to pieces again when fitting the new Piston, seal and pads.
-
Thanks Ted. Will do as you suggested when I take it to pieces again when fitting the new Piston, seal and pads.
IIRC there are some small seal rings fitted in the pivot.
-
Prompted by this discussion, I thought I'd check my front brake pivot (CB750K6), as I'm getting some slight binding and squeal, with a new calliper, fluid and pads. Sure enough, the pivot was not very free. I have managed to significantly improve it, with an hour's worth of Plus Gas and wiggling.
I can see no way of separating the pieces. The top and bottom brackets do not seem to unscrew from each other. They are solidly stuck in the same plane. Is there a technique for separating the two, so I can get to the pivot pin?
This piece: https://www.cmsnl.com/honda-cb750k6-four-england_model14383/pin-caliper-hold_45116323020/ is seized in the top mount (with the two holes to attach to the fork leg) https://www.cmsnl.com/honda-cb750k6-four-england_model14383/joint-caliper_45104317700/
Thanks, Martin
-
Unbolt the complete assemly from the fork slider 2x 10mm head bolts and 1 big one.
Turn the pivot so you can support the "ears" of the top pivot on the jaws of a vice and tap the steel pin out allong with pivot then clamp the flat faced bottom of pin in vice ans start wiggling and pulling moving bracket upwards till its off.
Clean the pin and bore of swinging piece, liberally copaslip and rebuid
-
Whilst you are that far in it's worth replacing the top & bottom O rings one of mine was split.
When I dismantled mine I had some green left over O rings from my Jeep air con repair kit - they were a perfect fit!
-
Unbolt the complete assemly from the fork slider 2x 10mm head bolts and 1 big one.
Turn the pivot so you can support the "ears" of the top pivot on the jaws of a vice and tap the steel pin out allong with pivot then clamp the flat faced bottom of pin in vice ans start wiggling and pulling moving bracket upwards till its off.
Clean the pin and bore of swinging piece, liberally copaslip and rebuid
That did it. Thanks.
Quick clean, rub over the pin with one of those nylon abrasive pads and a polish. All back together, with new o-rings and coppaslip. Had to relieve the brake pressure to get the calliper back on, quick bleed and good. Test ride at the weekend
-
Thanks all. I will certainly remove the pivot pin and follow the same procedure when I rebuild the caliper once the parts arrive from CMSL. But the difference that my temporary fix has made is amazing. Since getting the bike I have really struggled to move it around the garage as it seemed soooo heavy, and I have put about 1,000 miles on it, I’m pretty sure that all that time it has been dragging on that static pad. Took it out for a run yesterday and the difference was significant. I can also move it around the garage with ease. All because of a gummed up pivot pin!
-
I have written up a service schedule, largely based on the factory workshop manual. I'm going to add in giving the pivot a clean and re-grease every 2 years.
It is quite a vulnerable location, so im surprised this isn't observed more often.
-
People have been known to drill and tap the pivot arm and fit a grease nipple
-
I have written up a service schedule, largely based on the factory workshop manual. I'm going to add in giving the pivot a clean and re-grease every 2 years.
It is quite a vulnerable location, so im surprised this isn't observed more often.
Good Idea Martin, Is this service schedule something that you could share?
-
Happy to. Is there a way to upload an Excel spreadsheet on here?
P.s. just noticed under my forum name it now says 'SOHC Expert'. Nice, but would fail the Trade Descriptions Act :)
-
Also thanks to the OP @ColinHH. I did a test ride this morning and the brakes no longer bind or squeal. The pivot pin was my issue. Front brakes still lack any feel or real punch though!
-
Also thanks to the OP @ColinHH. I did a test ride this morning and the brakes no longer bind or squeal. The pivot pin was my issue. Front brakes still lack any feel or real punch though!
These are 50years old almost 1st generation disc brakes, good in the day,not as good as modern units,
They should improve after about 1,000miles usage, when the pads bed in
When you ride it ,and you're cruising along, lightly apply the brake, it will help it bed in
Sent from my SM-A546E using Tapatalk
-
I'm about 300 miles into the pads. I'll give it a go. 👍
-
I've always found the best way to bed pads in is to lightly drag the brake on and off a few times to warm the disk up and then get a bit of speed up and don't be afraid to pull the lever hard a few times.
Maybe do a few cycles and there as good as they will even be.
You will never go over the bars. But bedded in well and using the gearbox they stop ok.
-
Whilst my old XJ900 had brilliant disc brakes most of my previous bikes had state of the art back in the day 1960s drums. So my expectations of the 400 were pretty low - I have found them adequate provided my anticipation skills are switched on with no emergency stops required.
-
My biggest problem with 70's brakes isn't normally the front it's the rear drum. In that slight panic moment when you realise you need to stop suddenly. At a roundabout normally and you jab it to hard and lock up. For some reason it's really hard to tell your self to ease off
-
Whenever I swing my leg over the saddle, I go through a mental routine! i.e. I am on a 50 year old bike, and not my 2018 BMW, allow for a longer stopping distance, use the front and rear brake appropriately together with the gearbox to slow the bike down, Don’t expect it to stop! And I will mentally keep reminding myself during the ride.
-
Well said Colin.
-
I've always found the best way to bed pads in is to lightly drag the brake on and off a few times to warm the disk up and then get a bit of speed up and don't be afraid to pull the lever hard a few times.
Maybe do a few cycles and there as good as they will even be.
You will never go over the bars. But bedded in well and using the gearbox they stop ok.
I've tried this and it's worked. Reasonable front brake now. Thank you.
-
The feel and feedback from the front brake is not very good on these bikes, not the best design. However, what helped me was to find a very quiet road and do some emergency stop tests at different speeds using a fixed point at the side of the road and seeing how long it took to stop. I was surprised that the actual braking distance was much better than I had thought. Your bike should stop and if it doesn't, go over all the braking components again and check. Also, getting the adjustment right is very important. Annoyingly, on my bike, and I don't know if this is the same for everyone but, when my brake stops squeaking, I know it's maintenance and adjustment time.
Also, I have recently put on a replacement master cylinder and this seems to have improved braking performance a lot.
-
Agree when the squeak stops it's normally due a tweek.
-
I change my front brake to twin disc much better
-
Lòoks good. I thought about a twin disc, as the threads are there on the RH fork. What did you use for the pivot arm, as they look to be handed for the LH side only?
-
on the 500,the pivot is handed,but if I remember its only for the adjuster screw,when I fitted my 2nd disc I simply use a countersink drill bit to modify the arm,so the screw head would fit
-
Pauarc's and mine are 750s.