The theory of brake pressure v cylinder bores etc...

[Adrian Southgate]

I have been searching for new rear wheel cylinders for XSC in the shop parts catalogues and have come up with an issue.

There are two main bore sizes, 17.8 and 20.64 mm.

Assuming a constant fluid pressure and displacement from the master cylinder (X) am I right to assume the following.

17.8mm bores will effect more free travel at the piston due to smaller piston diameter but exert less pressure on the shoe?

Meaning if I want to improve the stopping power of the brakes, go up to the 20.64's and increase the displacement of the master cylinder to match?

I haven't measured the old ones and was wondering if I should just go with RS spec parts all the way through?

[James Doulton]

Yes, you are right. A larger bore means that you get more force for the same fluid pressure but it will require more fluid for the same movement so either the master cylinder will need to increase too or the brake pedal will move further. If you increase the bore of the master cylinder then the pedal force will rise to achieve the same pressure. Changing the rear brake cylinder bore while leaving the fronts the same will affect the brake balance front to rear.

[Adrian Southgate]

Hmmm, nothing there to change....It was drum fronts...

So, seems as if I should pick a model / year and just order all the parts to fit that car, either a mex or RS spec setup.

[Paul Sheridan]

Ade there is a good post on Mr G's on this subject written by Andy Smith , I  followed his recommendations when I was building my pedal box. 

[Big Vern]

What size drums do you have 8 or 9 inch?

[Adrian Southgate]

I'm 90% sure they are 8 inch mk2's but as I'm going round now to drop the diff into the axle casing i'll have a measure. I still have a complete back plate for each side fully assembled for reference as these were yet another different setup to any I have bumped into before... 

I'll take some before and after pics.

[Adrian Southgate]

8" ID drums - backing plate with all the fittings looks like this...., this is the nearside backing plate and its in the same orientation in the photo as it will be on the car.

I do have a full set of 9" drums, shoes and backplates but was saving those for the LSD axle. Just wondering if the 20mm bore slave cylinders are for the 9 inch rears? Maybe I should strip one of the old manky ones on this back plate and measure it.

[Steve Kerswell]

My Sierra with 8" and my B+ with 9" are both as your orientation suggest. The Hood Sat on my drive has a Atlas and they are at 6o'clock.

[Big Vern]

Its very important not to over brake the rears. When you stamp on the brakes it's the fronts that should lock. Only over brake the rears if you like hospital food. If you're using type 16 calipers on the front then go with whatever rear cylinders were used with t16 fronts and 8" drums, or whatever cylinders were used with t16 fronts and 9" rears. Brakes international is a good place to check up on what each configuration had as stock.