40 replies to this topic

[CDY]

One other question Spider, if I may. What size should the rear brake cylinders be if the original correct system for an April 1980 1275gt was a split two pipe front to back system with a GMC173 Master Cylinder and a FAM7821 Pressure Limiting Valve on the Bulkhead?

Also would it just be a “T” connector on the rear subframe?

[Spider]

One other question Spider, if I may. What size should the rear brake cylinders be if the original correct system for an April 1980 1275gt was a split two pipe front to back system with a GMC173 Master Cylinder and a FAM7821 Pressure Limiting Valve on the Bulkhead?

Also would it just be a “T” connector on the rear subframe?

 

Wheel Cylinders - my Fiche is not in great condition, but doing my best here;-

 

To Chassis No 19415A                      GWC1101    (5/8")       These the original Single Circuit with 7.5" Discs
From Chassis 19416A to 21569A     GWC1129     (11/16")   These were fitted we when they went to 12" wheels and 8.4" Discs (Single Circuit)

From Chassis 21570A                       GWC1126    (1/2")        With the change to Tandem Brakes

 

The FAM7821 was fitted from Chassis No. 40924A

 

So, I'd be reasonably sure yours would have the FAM7821 and 1/2" Rear Wheel Cylinders.

 

Yes, there's just a Tee on the rear Subframe.

[CDY]

Hi Spider

 

My VIN number is XE2D2000717146 so I am not sure which part relates to my Chassis number. Can you enlighten me?

 

Thanks

[Spider]

Hi Spider

 

My VIN number is XE2D2000717146 so I am not sure which part relates to my Chassis number. Can you enlighten me?

 

Thanks

 

Relating the above numbers to yours, if you drop the prefix, yours is 717146 and I take it yours is RHD, so there's be the letter A after that.

[CDY]

Thanks Spider.

 

If my chassis number is 717146(A) and they changed to Tandem Brakes (front and back) from Chassis 21570A (with rear wheel brake slave cylinders being GWC1126 (1/2")), they must have produced quite a large number in just a few years.

 

I think the 8.4 discs were introduced around 1976(?) when they were only on chassis number 19416A.

 

This would mean, if I have my dates about right, that between 1976 and 1980 (when my car was built), they produced nearly 700,000 new chassis.  (717,146 less 19,416).
 

Did they by chance miss out a big chunk in the numbering or is the above correct?

 

Perhaps I have missed something, but I am a little intrigued.
 

[Spider]

I don't know for sure, I can only work from what's in the Fiche. I'm sure that the numbers don't relate to the GT alone, but across all Minis. They may have jumped a few hundred or thousand at a certain point when there was some other significant and likely legal change.

[CDY]

Hi Spider. Sorry to bring this up again.

I have just looked on Mini Spares data and they agree that a 1980 1275GT should have tandem front to back brakes, the GMC227 master cylinder (the only replacement for the original GMC173), a FAM2781 regulator valve and apparently GWC1102 3/4" rear brake cylinders (not the 1/2" ones which were used when no regulator valves were installed up to 1978).

 

See the red circled area below.

 

I would be interested to see what you or anyone else thinks, prior to changing mine back to the correct system. 

 

 

 

Attached Files

•  brakes.JPG   21.63K   0 downloads

[Spider]

and apparently GWC1102 3/4" rear brake cylinders (not the 1/2" ones which were used when no regulator valves were installed up to 1978).

 

See the red circled area below.

 

I would be interested to see what you or anyone else thinks, prior to changing mine back to the correct system. 

 

The cylinders and sizes I posted up ^ were from the part's listing fiche.

 

They also sound / look right to me.

I know from experience that when having any factory Discs on the front, generally on any Mini the biggest rear cylinder you might be able to get away with and avoid locking up the rears might be 11/16" though that only works in some circumstances. Most Minis require 5/8" Cylinders or if there's nothing to shut off or regulate the pressures to the rears, then 1/2" cylinders.

You could try the 3/4" cylinders, however, when you have the car back on the road, test them in a safe area on a wet road or gravel.

[CDY]

Hi Spider. Sorry for the time delay for my reply (over a month) but I was doing some further investigation.

 

Mini Spares in London came up with a good point regarding the 3/4” rear cylinders they have on their chart on the web. They said that the larger the cylinder bore then the more fluid it requires to activate the cylinder, then the brakes. Based on this the smaller cylinders are more likely to lock up than the larger cylinders.

 

There does seem to be a certain logic to this.

 

They are still saying that the correct kit is as previously detailed, i.e..

 

“I have just looked on Mini Spares data and they agree that a 1980 1275GT should have tandem front to back brakes, the GMC227 master cylinder (the only replacement for the original GMC173), a FAM2781 regulator valve and apparently GWC1102 3/4" rear brake cylinders (not the 1/2" or the 11/16” ones which were used when no regulator valves were installed up to 1978)”.

 

I am therefore slightly confused as to which size cylinders to use to reproduce the original, with a FAM2781 regulator and the GMC227 master.

 

can you or anyone else shed any more light on this please?

[Chris1275gt]

The ones quoted above 3/4" as you have a regulator fitted. I have a 75gt single line, no regulator and 1/2" cylinders, when I got it someone had put the 3/4" cylinders on and the rears locked every time, so the regulator with the 3/4 cylinders regulates the pressure to stop them locking up and allows more pressure to the fronts. My logic is telling me that 1/2" cylinders would lock with a regulator fitted but the 3/4" cylinders would be fine?

[Spider]

Perhaps a few fundamentals regarding Hydraulics might be in order ?

 

Considering the Cylinder that receives the Mechanical Input (ie in this case, the Master), when going from a smaller to bigger dia, it displaces more fluid for the same movement, equally, do develop the same Hydraulic pressure a higher mechanical force input is needed (you need to stand on the pedal harder).

 

Considering the Cylinder that Outputs to a Mechanical Movement, when going from a smaller cylinder to a bigger one, as it has a bigger surface area on the Piston, it needs more fluid to move the same distance, but as it is bigger, it develops a higher force Output from the Piston.

 

So, coming back to rear brakes, a smaller cylinder moves more but will press 'softer' on the shoes compared to a bigger cylinder. Further, because the smaller cylinders will move further (more), this translates to less movement of the pedal (a 'higher' pedal). In other words, a smaller cylinder is less likely to lock up the brakes than a bigger one.

In regards to whether the cylinder sizes should be 1/2", 11/16" or 3/4", the info I've passed on here is not only from the parts books, but also in my own experience, a Mini fitted with either 7.5" or 8.4" Discs on the front and 3/4" rear Cylinders will result in the Rears locking up.

 

Here's some factory data, sorry that it only covers early cars, however note the rear wheel cylinder sizes between the models that have drums up front to those with discs;-

 

 

Wheel Cylinders aren't expensive nor difficult to replace. Rather than go of the words / charts / tables between myself and Mini Spares, but a set of 1/2" cylinders and 3/4" cylinder. Try them both in similar conditions (preferably adverse).