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MG MGB Technical - GT Rear Brake Cylinders

I'm preparing a "new" rear axle to replace the one in my '67 GT. The new axle came out of a '73 GT. If I want to replace the brake hydraulic cylinders, which ones should I use? Moss lists one with .800" bore for early GTs and a different one with .875" bore for later GTs. My chassis number puts me in the early lot, but the axle is later. Thanks. -G.
Glenn G

Personally, I would be inclined to stay with the smaller bore. If you do, you will need to drill a new hole in the backplate for the locating pin, as it is in a different position.
Dave O'Neill2

Glenn-
The MKII GT modcels used the larger .875" bore slave cylinder in order to compensate for the additional weight on the rear of the GT model. As Dave points out, the locating hole in the brake backplate for the later slave cylinder is in a different position. If the 1973 MKII GT rear axle that you're planning to install came with the brake backplates, then performance-wise you're better off using the later slave cylinder.
Stephen Strange

I had a similar situation putting a later GT axle into a roadster. I welded the old holes and drilled new so I could use the correct slaves. However in your case the later slaves being bigger will give more retardation at the rear. But it's only an issue if changing the backplates as well, which I did, wanting to clean and paint the ones on the replacement axle rather than have the car off the road while I did the originals.
PaulH Solihull

So it seems I can choose which cylinders to use if I have the early backplates or I'm able to modify the later backplates.

The car currently has a later model axle, but I don't remember which backplates or cylinders I used (that was seven years ago). Thanks for pointing out the differences.

I wonder if it would affect the operation of the handbrake if I used larger cylinders. I've always had difficulty getting the handbrake to hold. -G.
Glenn G

for what it's worth Factory V8's used the smaller wheel cylinders

Bob
Bob Fisher

Bearing Bob's advice about the use of roadster cylinders on the more powerful V8, I would incline to Dave's view and be leary of more powerful rear brakes. If the handbrake doesn't hold the car, either the rear brakes were not correctly adjusted or the handbrake cable has stretched - more easily put right.
FWIW.
Roger
R Walker

The handbrake is nothing to do with the wheel cylinders, it acts directly on the shoes through cables and levers.

Bl handbrakes of the era are notoriously poor, but that is because people rarely bother to strip, clean and lubricate them on a regular basis. I do mine annually regardless of mileage and my MOT has expressed his amazement more than once. I can't lock them going down a dry road (and neither should you be able to), but I can when I roll the cars out of the garage down the slight slope then lean in and pull up the handbrake. I've parked both cars on some considerable slopes in the past (like half-way up the Hardknott Pass) and they have held well. Of course had the cable chosen that time to snap it would have been a different matter ...

Stretching of the handbrake cable does occur over time, but again doesn't affect the ability of the handbrake to hold the car on a slope, unless you have run out of adjustment and simply can't pull the cabin lever back far enough. My V8 cable (which came off my roadster when I changed that to a wire wheel axle) ran out of adjustment, so I put a 1" clamp onto the end of the short cable at the compensating lever on the diff which corrected that.

PaulH Solihull

Yes, I'm planning to adjust the drum brakes and the handbrake cable before the state inspection due in December. The handbrake does work a lot better after adjustment. Probably should make that an annual ritual. The car has never failed an inspection, but our inspectors are not very strict.

I had a Sprite once that reminded me about the handbrake adjustment one day by getting jammed in the full-on position! -G.
Glenn G

This thread was discussed between 26/11/2012 and 28/11/2012

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