CB750F1 Rear Disc Brake

[Johnwebley]

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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[hairygit]

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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[Doswen]

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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[Doswen]

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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[Doswen]

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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[Spitfire]

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

[Moorey]

Personally I don’t see just removing a part that should be there, as a cure for the problem.

[K2-K6]

Wondering if anyone has a detailed close up picture of that "device" so we can try and figure what they've done in there.

[Doswen]

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.

[Doswen]

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