Tandem brake master cylinder

[Dave Plowman (News]

This is what the one on my old car is described as. Has two outputs for dual circuit brakes. One takes a 12mm male union, the other a 10mm as used pretty well everywhere else on the car. However, the pipe size is the same

3/16ths to both.

I had a pipe rust through. So decided to replace them all. And got a kit of ready mades, specific to the make/model.

Only one of the supplied pipes has a 12mm male - as you'd expect. But this is a long pipe which crosses the car to the LH front caliper - one of two circuits.

The original pipework on the car shows this large outlet going to the balance valve underneath the master cylinder, so a short pipe. No possibility of mixing the pipes up due to the vastly different lengths.

The makers of the kit say it makes no difference if the two are swapped. If that were the case, why two different outlet threads? Which prevents them being swapped?

I don't really understand how a tandem cylinder works. Any experts say for certain if swapping them doesn't matter?

[Steve Walker]

They are probably correct. The tandem cylinder simply has two, loosely connected, pistons in it.

In normal use, pressing the pedal pushes the rearmost piston, which pressurises the fluid between the pistons (the rearmost connection is tapped from here). The pressurised fluid works one circuit and also pushes the second piston, which pressurises the fluid ahead of it (the forward connection is tapped from here).

Both operate together, both reach the same pressure.

If one circuit leaks, no pressure will be produced in it.

If the front tap-off has no pressure, the front piston will hit the end of the cylinder before it stops moving and the rear piston can then raise the pressure between the pistons. The rear tap-off will still work, but the pedal will travel further before having any braking effect.

If the rear tap-off has no pressure, the rear piston will hit the front piston and push it along, without raising the pressure between the two. The front piston can then raise the pressure in front of it. The front tap-off will still work, but the pedal will travel further before having any braking effect.

[newshound]

Useful explanation. I recently had a total brake failure on an ATV with dual circuit brakes, but only found one burst hose. I'm not sure whether I have an internal problem in the master cylinder, or whether I lost enough fluid on my 35 degree slopes to let the other circuit run short too. Working at the moment and the master cylinder is a bugger to get out, but useful to know what to look for if I do have to check it.

[Animal]

I can't answer the op's question in practice, but from theory I know the pipe dia will make no difference other than, if overly narrow, to slow the application of brakes a miniscule amount. But I am familiar with the problem of dual brake systems suffering 100% failure due to one circuit failing, and the description here fits my vague recollection of how such systems worked.

[Dave Plowman (News]

Thanks Steve. Just wondered why they used different fittings if the order didn't matter.

[Steve Walker]

Who knows. It generally doesn't.

On one of my cars, the rear connection is a single and has a tee-piece at the back of the car, while the front has two separate connections, but an alternative version of the master cylinder just has two, single connections and a screw-in tee for the front.

[Dave Plowman (News]

I decided to make up new pipes with the correct unions. And glad I did. Now it's all back together. If they had been fitted crossed over, it would look rather strange (to me) as all the pipework is bent to be either vertical or horizontal, where it can be. And they would then both have been at an odd angle. And possibly rub together.