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Rear Wheel Cylinder Issue Question / 7.5'' Brakes


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#1 rider4life

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Posted 22 February 2018 - 05:49 PM

Converted my 1980 clubman from drums to a 7.5'' conversion using original cooper s discs/calipers, fitted goodridge lines all round aswell, No servo on a split system, Brakes are not great for a start, but if i emergency stop the fronts will never lock up but the rears do, Even if im in traffic and a little firm on the brakes. 1 rear wheel will skid. 

 

What rear cylinders shall i buy that might help the situation?



#2 hhhh

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Posted 22 February 2018 - 05:57 PM

Sounds like it might be your balance valve is set too hot towards the rear. If it's the ball up the ramp type you can adjust its angle, but if its the pressure set point type, you'll need to replace it. That aside, if only one is locking up, your rears are imbalanced. I'd check shoe adjustments. Another concern is that the drum master cylinder may not have a large enough reservoir for discs.


Edited by hhhh, 22 February 2018 - 05:59 PM.


#3 Stu1961

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Posted 22 February 2018 - 07:42 PM

I think the rear cylinders should be GWC1101, 5/8" bore



#4 carbon

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Posted 22 February 2018 - 09:28 PM

Which bore size rear cylinders do you currently have, and is there a pressure limiting valve fitted?

 

As above, would expect them to be 5/8 inch bore with 7.5 discs up front.



#5 rider4life

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Posted 22 February 2018 - 10:23 PM

I have no idea what rear cylinders are on it, No pressure limiting valves fitted



#6 carbon

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Posted 22 February 2018 - 10:32 PM

It's really not safe to drive as it stands. You have a non-standard brake set-up that has too much braking on the rear.

 

If you have to brake suddenly you run the risk that both rear wheels lock up, and then the car can spin. This can happen very quickly.



#7 timmy850

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Posted 22 February 2018 - 11:30 PM

Minispares have a list of the factory installed combinations - using a known combination would be a good idea

http://www.minispare...rt.aspx?1~9~105



#8 mikal

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Posted 23 February 2018 - 01:52 AM

As people have said, you need the smaller diameter brake cylinders on the rear otherwise you will experience exactly as you have described. Over braking, lock up at the rear..



#9 hhhh

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Posted 23 February 2018 - 02:32 AM

The rear brake cylinder diameter doesn't matter if the balance valve is installed and correctly configured.



#10 mikal

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Posted 23 February 2018 - 06:29 AM

He said he doesn't have brake pressure limiting valve installed....

My Mk1 has Cooper S discs, rear drums and cylinders fitted. No valve either (what year were they introduced?). The brakes are very good and I've never locked up the rears.... (Tested it out doing full on emergency braking tests).

When I installed the disc brakes eons ago I was told I MUST install the smaller diameter brake cylinders to prevent exactly what the OP is describing...



#11 timmy850

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Posted 23 February 2018 - 06:56 AM

So when you say there is no pressure limiting valve, do you mean that you removed it or it's not adjustable?

 

A 1978 onwards has this part listed

http://www.minispare...ic/FAM7821.aspx

 

Mikal, your car has one on the rear subframe and would look like this

http://www.minispare...ic/21A1774.aspx


Edited by timmy850, 23 February 2018 - 06:57 AM.


#12 mikal

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Posted 23 February 2018 - 08:35 AM

Hello Timmy,

 

My car (1967 Oz) only has a simple T piece bolted to the front of the rear subframe, no adjustable device like the one you referred to. It was when I noticed that it concerned me so I did the heavy braking tests. The brakes work well so I didn't investigate it further. Perhaps I should, I really don't know....



#13 timmy850

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Posted 23 February 2018 - 09:36 AM

Hello Timmy,
 
My car (1967 Oz) only has a simple T piece bolted to the front of the rear subframe, no adjustable device like the one you referred to. It was when I noticed that it concerned me so I did the heavy braking tests. The brakes work well so I didn't investigate it further. Perhaps I should, I really don't know....

Every car was fitted with one. It might look like a simple T junction but they regulate the rear pressure too
https://minikingdomo...er-or-cooper-s/

#14 mikal

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Posted 23 February 2018 - 09:54 AM

Hi Timmy,

 

I hear you but my car has only a simple T piece. (Due to my work in a previous life I know a T piece when I see one). Why I have only a T piece I have no idea. I'll research it here and try to find out why... Maybe a previous owner(s) installed it? The car is 50 years old.. The odd thing is, looking at it and the design (brass, looks 50 years old..), it looks like an original bit of equipment made to fit where it is. I don't know...

Having said that, the brakes work very well as they are. A mystery...  



#15 hhhh

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Posted 23 February 2018 - 01:21 PM

I wouldn't drive a Mini without a limiter fitted. I've had rear brake lockup and it's not fun. As previously mentioned, it spins around faster than you can react.






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