Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Doswen on October 09, 2020, 11:04:02 AM
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The rear caliper piston won’t retract leading to overheating.
I’ve been on this problem for a number of days now and can’t solve the problem - help please.
I’ve tried two different rebuilt callipers.
New HEL braided oil line with new banjos. The banjo bolts are shorter than the originals.
Tried two different seal kits in the master cylinder. Now had it apart 4 times.
The return hole is clear.
The only way the pistons will push back in is by undoing the bleed valve or the banjo at the master cylinder.
The brake works releases fine by hand but if it is run up and gets hot the brake binds badly and overheats.
To be honest I can’t see how the seals in the master cylinder release the pressure when they both face the same way and the cup on the end of the spring is over the outlet hole.
I don’t know what else to try.
I also rebuilt the front brakes (twin disc conversion) and have no problem with those.
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After you have fitted a new caliper seal and bled the system..
Operate the pedal like you are braking. The piston should move out. Then as you release the pedal it should retract slightly.
Check it moves freely
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To add to John's comment, make sure the outside metal edge of the pad is clean with no rust as this can sometimes just cause enough friction between the pad and the inside face of the caliper body to prevent the pad from fully retracting.
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Is it a F1 seal kit as they can be very hard to find now.
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After you have fitted a new caliper seal and bled the system..
Operate the pedal like you are braking. The piston should move out. Then as you release the pedal it should retract slightly.
Check it moves freely
John, the piston moves out and retracts enough to free the disc when the wheel is turned by hand.
If the braking is under load with the engine running it will get progressively hotter until is completely locks up.
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To add to John's comment, make sure the outside metal edge of the pad is clean with no rust as this can sometimes just cause enough friction between the pad and the inside face of the caliper body to prevent the pad from fully retracting.
Morey, the pads are new and there is no rust. The bike has just undergone a 2 1/2 year refurb.
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Is it a F1 seal kit as they can be very hard to find now.
Dave, it is an F1 and yes the kit was difficult to find in the UK. I had to order the GL1000 kit From the USA and cut down the plunger thread. The kit is otherwise identical but I have also tried another set of seals from another F1 master cylinder and it’s no different. Still locks up under load.
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It is "classically" a restriction of the return route into the mastercylinder.
If all the caliper works ok and is demonstrably free, then it's just the fluid that's holding the pads from retracting. From then on it'll heat fairly quickly.
Are you certain that you've seen completely clear passages within the mastercylinder? I'd be surprised if it's not within there.
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Is it a F1 seal kit as they can be very hard to find now.
Dave, it is an F1 and yes the kit was difficult to find in the UK. I had to order the GL1000 kit From the USA and cut down the plunger thread. The kit is otherwise identical but I have also tried another set of seals from another F1 master cylinder and it’s no different. Still locks up under load.
No idea how much you cut off the plunger thread but i had to cut 6mm off the GL plunger and it worked fine.
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It is "classically" a restriction of the return route into the mastercylinder.
If all the caliper works ok and is demonstrably free, then it's just the fluid that's holding the pads from retracting. From then on it'll heat fairly quickly.
Are you certain that you've seen completely clear passages within the mastercylinder? I'd be surprised if it's not within there.
I‘ve had a cocktail stick through the return hole and blown it with an airline. I’ve checked it 4 times now !
I just can’t work it out.
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Is it a F1 seal kit as they can be very hard to find now.
Dave, it is an F1 and yes the kit was difficult to find in the UK. I had to order the GL1000 kit From the USA and cut down the plunger thread. The kit is otherwise identical but I have also tried another set of seals from another F1 master cylinder and it’s no different. Still locks up under load.
No idea how much you cut off the plunger thread but i had to cut 6mm off the GL plunger and it worked fine.
I matched the thread length to the original.
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Also classic of hardened crap in the caliper seal groove.
Ive had them where a dremel brush just polishes the crud, you need to use a dental pick and change the seal
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If you put a clamp on one piston and squeeze it backwards into it's bore, then does the other piston come out, or does the fluid come back into the reservoir?
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Also classic of hardened crap in the caliper seal groove.
Ive had them where a dremel brush just polishes the crud, you need to use a dental pick and change the seal
Yes was cleaned out with a pick and yes had new seals. The seal isn’t leaking. Also fitted another rebuilt caliper also with new seal and had the same problem.
So I don’t think the caliper is the culprit but I don’t know what is.
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If you put a clamp on one piston and squeeze it backwards into it's bore, then does the other piston come out, or does the fluid come back into the reservoir?
Neither piston will noticeably push back. I have to open the bleed valve to get them to move.
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Ideally both pistons should push back into the caliper.
Make sure they can slide.
But it seems like an issue with the returning fluid into the master cylinder.
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Many years ago I had a similar problem with an F1 rear caliper. In the end after discussing with a local Honda dealer, he suggested re assembling the master cylinder but WITHOUT the restrictor valve (the little black domed bit of metal sitting at the top of the spring in your photo. I did just that, problem solved. A word of warning though, the restrictor is there to reduce the amount of pressure reaching the pistons, as the rear brake without the restrictor is incredibly powerful, and if you're not careful you can lock the rear very easily under braking. That said, I learnt very quickly to use it very gently, as 80% of your braking should be on the front anyway. Try it, if it works, it's your choice to either search out another restrictor valve or run without it.
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Both pistons slide freely with the bleed nipple open. I agree it all points to the pressure not being released in the master cylinder, however I can’t find the fault.
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Aaarrrhhh, that’s what the dome cup is. I have swapped that too and it made no difference.
It sounds like a good idea to remove it and see if it cures the problem. Many thanks.
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Many years ago I had a similar problem with an F1 rear caliper. In the end after discussing with a local Honda dealer, he suggested re assembling the master cylinder but WITHOUT the restrictor valve (the little black domed bit of metal sitting at the top of the spring in your photo. I did just that, problem solved. A word of warning though, the restrictor is there to reduce the amount of pressure reaching the pistons, as the rear brake without the restrictor is incredibly powerful, and if you're not careful you can lock the rear very easily under braking. That said, I learnt very quickly to use it very gently, as 80% of your braking should be on the front anyway. Try it, if it works, it's your choice to either search out another restrictor valve or run without it.
Well Hairy Git, it looks like you cracked it. I’ve removed the restrictor valve and it seems to have cured the problem. Thank you so much.
I’ve run the bike up on the centre stand with heavy braking under load and there are none of the old binding symptoms. Waiting for the roads to dry out and then I can give it a road test but all the signs are looking good.
As per my original message I can’t really see how the restrictor valve worked when it blocked the oil way. The brake doesn’t seem any sharper than with it in but I will be able to judge it more on the road.
Thanks again 👍😀
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The back brake on my F1 always amazes me, it is so much stronger than the front brake, I always wonder why they did it that way.
Cheers
Dennis
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Personally I don’t see just removing a part that should be there, as a cure for the problem.
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Wondering if anyone has a detailed close up picture of that "device" so we can try and figure what they've done in there.
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It’s this domed cup that sits over the flow/return hole.
I’ve had no problems since removing it and the brake is no sharper than it was with it in.
I know it worrying removing a part from a braking system but in comparison the front brake doesn’t have a similar restrictor.
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If you look at the close up of the cup it has a slot about 1mm wide in the circumference. As far as I can work out this is the only passage for the oil pressure to be released