Binding brakes this time.

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Bikergonebald
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Binding brakes this time.

Post by Bikergonebald » Mon May 07, 2018 8:03 pm

In the last two weeks I’ve had to fix power steering leaks and have the radiator recored. A quick test drive revealed the hot weather had freed up the seat runners but the latch mechanism didn’t work so I had to take the seat and and fix that but it should be good for another 40years.

The next test drive got me 15 miles before I realised the rear brakes were binding. The pedal firmed up so there was no free play and it didn’t roll when stopped on a gradient. From my knowledge of binding brakes it could be; master cylinder (which is only a year or two old), collapsing brake flexies, slave cylinders or balance valve.

Any suggestion on where I should start?

And by the way the power steering pipe is still leaking.


Kevin

Scimitar Se6a 1979
Honda CB900c

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Binding brakes this time.

Post by Oldconn » Tue May 08, 2018 12:21 pm

I would look at what you are calling the balance valve . Mounted where they are on the chassis, they suffer a lot from water ingress & corrosion.


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Binding brakes this time.

Post by Coupe Racing » Tue May 08, 2018 12:47 pm

Are the rear brakes over adjusted?
Have you taken pedal free play out by adjusting the push rod ( affects all wheels)
Both can cause brakes to bind


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Binding brakes this time.

Post by Old and Slow » Tue May 08, 2018 1:40 pm

I think there's a thread on here somewhere that describes your symptoms, the pedal creeping up and becoming solid. IIRC it has something to do with a "relief valve" in the master cylinder in servo-assisted systems. Hope you can find it.
Philip


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Bikergonebald
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Binding brakes this time.

Post by Bikergonebald » Wed May 09, 2018 12:39 pm

Thanks guys.
I found a thread about the front brakes binding and the conclusion for that seemed to be incorrect adjustment of the master cylinder preventing it getting back to the position which allows fluid to return back up to the reservoir. I had assumed it was the back brakes binding but will jack it up and check. I've never done any work or even servicing on the brakes because i've not had it long. The previous owner had replaced a lot of brake parts including the master cylinder so it could be that.
Hopefully i will get time at the weekend to investigate. We;re 5 months in to the year and so far i've only be able to burn off one tank of fuel.


Kevin

Scimitar Se6a 1979
Honda CB900c

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Binding brakes this time.

Post by Nigel Clark » Wed May 09, 2018 3:16 pm

If you're sure it's only the rear brakes binding, I would suggest checking/replacing things as follows, starting with the easiest:

1. Rear brake adjustment.

2. Handbrake cable adjustment.

3. Condition of brake shoes and particularly return springs. Have the springs been fitted correctly?

4. Replace flexi-hose to rear brakes (single hose above the axle).

5. Brake balance valve.

If the front brakes are also binding, suspect master cylinder push rod adjustment as mentioned, or possibly a servo fault.

Hope this helps,

Nigel



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Binding brakes this time.

Post by Bikergonebald » Thu May 10, 2018 8:44 pm

I’ve had a better look now. One of the rears was binding but that was just bad adjustment, the other side was ok. I was doing this cold and the front brakes were ok. I went for a drive and could feel the pedal travel reducing.
When I jacked it up with the brakes hot both rears were ok and it was the fronts that were tight.
So it could be calipers or flex hoses but it’s unlikely that both sides would be affected the same.
It could be the balance valve but I read on an SD1 forum that they only tend to cause problems when they leak - but I’m not ruling it out.
The most likely culprit to me is the master cylinder not letting fluid back up to the reservoir when it gets hot and expands.

In the car there is a small amount of free play between the rod from the pedal and the master cylinder but how much should there be? Or could this free play be between the pedal and servo but there’s no play between the servo and cylinder?

The other possibility is a blockage inside the master cylinder. I’ve seen this on other cars with different cylinders but they were old and my master cylinder looks almost new.

So. Assuming it is the master cylinder where do I start trying to diagnose and fix it? The manual isn’t much help.

It’s a Lockheed system.

Thanks
Kevin


Kevin

Scimitar Se6a 1979
Honda CB900c

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Binding brakes this time.

Post by Bikergonebald » Sun May 13, 2018 9:48 am

Yesterday I got the brakes hot so they were binding, I slackened the nuts holding the cylinder to the servo and the brakes released.
There wasn’t anything to adjust so I put in a metal ring that was 1.8 mm thick. They didn’t bind but the pedal felt a bit low so I made up a ‘gasket’ from 1mm aluminium.
Now it all works fine.

Thanks guys.


Kevin

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Honda CB900c

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Binding brakes this time.

Post by AllingtonGT » Sun May 13, 2018 10:09 am

Glad its fixed the problem, but what do you surmise was happening to create the binding of the brakes?



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Binding brakes this time.

Post by Bikergonebald » Mon May 21, 2018 9:46 pm

When the pedal is released the master cylinder piston is pushed back to its rest position by the return springs in the back brakes and in the cylinder. In this position a small port is opened up between the reservoir and the fluid side of the piston, this allows a bit more fluid in to the system if needed or if the fluid has expanded due to heat it can return to the reservoir.
In my system the master cylinder piston rod was a fraction longer than it should have been or the master cylinder flange was too short (badly modified). This meant that the servo rod stopped the cylinder rod getting back to its rest position and allowing fluid to flow to or from the reservoir.
When the fluid expanded due to heat it had nowhere to go and so maintained pressure on the front brakes.
Putting the 1mm shim between the servo and cylinder allowed the piston to get back to rest.

Now it’s all good.


Kevin

Scimitar Se6a 1979
Honda CB900c

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Binding brakes this time.

Post by Bikergonebald » Wed Jun 27, 2018 8:31 am

The problem has come back!
After fitting the shim the problem had gone away and I was able to enjoy driving the car again but this week on a particularly hot day it came back leaving me stuck at the side of the road with smoking brakes so hot that water boiled off them. Returning after a few hours they had cooled and were dragging but I could turn them. I took the pads out and the pistons were free to move, the callipers are nearly new as are the flex hoses which are stainless steel braided, the solid parts of the brake lines are copper and again look quite new. I added an extra shim between the master cylinder and servo and drove home but after a few miles the brakes were dragging again so it can't be the clearance between the master cylinder and the servo causing it.

Any ideas?
I have ruled out;
Callipers - look new and free to move
Rear brake cylinders and mechanisms - new two weeks ago
Hoses - all flexibles are relatively new stainless steel braided

This leaves
Master cylinder - it's only a year or so old but I guess it could be faulty.
Brake pressure reducing valve - Could it cause these symptoms?

I really need help on this one, I can't use it for commuting again until I'm sure it's fixed.

Thanks.


Kevin

Scimitar Se6a 1979
Honda CB900c

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Binding brakes this time.

Post by scimjim » Wed Jun 27, 2018 9:27 am

Was there not a problem with a certain brand/type of mc sticking on a few years ago?


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Binding brakes this time.

Post by Rev Light » Wed Jun 27, 2018 12:27 pm

There was the replacement servo's staying on. Removal of one of the O-rings seemed to sure mine.

Have you changed your servo to an after market one - Lockheed?

Steve


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Binding brakes this time.

Post by Bikergonebald » Wed Jun 27, 2018 12:37 pm

I just found this on the triumph wedge owners site

https://www.triumphwedgeowners.org/uplo ... brakes.pdf

Page 11 of 16 it shows how to adjust the push rod between the servo and master cylinder. From what I've read before the servo could be the same between the TR7 and the Scimitar. even if not identical there's a good chance the construction is the same. Does anyone have a dealers service manual for the SE6a?

I'm not finding any suggestions on the internet that the pressure reducing valve could be at fault and looking at diagrams on how they work I don't believe it could be.


Kevin

Scimitar Se6a 1979
Honda CB900c

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Binding brakes this time.

Post by Bikergonebald » Wed Jun 27, 2018 12:41 pm

Sorry rev-light, I missed your post.

I don't think the servo has been changed but the previous owner has replaced the master cylinder, he said something about it being off a "rover" but was bought ready machined to fit. This is what made me look at the clearance between the two previously. If the servo staying on is a possibility then I think I need to get that off and take a look inside.


Kevin

Scimitar Se6a 1979
Honda CB900c

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