Since I?m also about to replumb my car as a front/rear system, I thought I?d better find out exactly how the Volvo master cylinder works. That SAE paper hiperfauto refers to above explains it all.
The Volvo 240 stepped bore master is very nearly the same as a normal tandem master, but with one key change (apart from the stepped bore, okay that?s two changes), an additional ?spacing sleeve? between the two pistons that prevents the secondary piston from moving too far away from the primary piston. The spacing sleeve is mechanically attached to both pistons, but with some movement allowed. This was introduced to prevent the sudden increase in pedal travel that would occur if a circuit were to fail in a standard tandem master cylinder system. You will also notice that the area of the large piston is exactly twice that of the small piston. This is important.
During normal use the primary piston (1) (22.2mm, nearest to the pedal) generates all of the pressure. The secondary piston (2) ?floats? just as in a normal tandem system with equal pistons. The floating piston equalises the pressure in both circuits (mostly, see aside 1). Since the 240 uses two identical circuits made up of a pair of pistons in each front calliper and one rear calliper, the amount of fluid displaced is the same in each. Piston 2 floats within the bounds allowed by the spacing sleeve. Everything works as normal.
If the primary circuit fails, that piston (1), unhindered by hydraulic fluid, moves forward and pushes against the secondary piston (2) via the spacer sleeve. This is fairly normal for a tandem master, except for the spacing sleeve. Pedal travel increases slightly and the pressure in the remaining good circuit is twice that of normal as the secondary piston is half the area of the normal working piston.
If the secondary circuit fails, the secondary piston is no longer floating between two chambers of hydraulic fluid, but it cannot move all the way to the end of the cylinder (as would happen in a normal tandem master cylinder) because it is restrained by the spacing sleeve. Because the primary piston is pushing twice as much fluid as the smaller piston 2 which is moving with it, the resulting fluid displaced is the amount displaced by the large piston (1) minus the amount displaced by the small piston (2). Since piston 1 is twice the area of piston 2 the amount displaced is the same as for just one of the smaller pistons, with the same result as for the previous failure case, the pedal travel increases slightly and the pressure in the remaining circuit is twice as much as normal.
Normal behavior depends on the volume of the two circuits being the same to allow the spacing sleeve to restrain the relative movement of the pistons within an acceptable range. There is a note in the SAE article that this type of master is okay on a diagonally split system (which the 240 triangular setup is, sort of), but would not work on a front/rear split system as it would be unlikely that the volume displaced in the front and rear circuits would be the same. Calliper piston retraction also has a part to play in this volume, so even if the callipers were the same volume, the actual volume displaced by the pistons retracting in two different types of calliper would be unlikely to be the same.
So what does this mean for rearranging the Volvo 240 system for a front/rear split?
Don?t. The stepped master cylinder is designed to work with two circuits which displace exactly the same volume of fluid. A front/rear split generally will not do that. It would probably work, but would behave unpredictably as it would be functioning as if one circuit had failed and therefore increase the pressure in the other circuit.
The stepped master cylinder can safely be replumbed to a standard diagonal split with alternative front callipers that have a single line, as thousands of people have done before, but
if going for a front/rear split I would change the master cylinder to a normal non-stepped version (also see aside 2 below).
I?m guessing that Volvo changed the master cylinder to a normal non-stepped one for ABS models, which are a front/rear split? But we didn?t get ABS on many 240?s in the UK so I?ve never seen one.
Aside 1
There is a note in the SAE article that due to friction and the spring acting on the secondary piston there is a slight difference in pressure in each circuit and it is therefore important to minimise king-pin offset to prevent pulling to one side. It doesn?t quantify this difference, but I would imagine it is very small, especially with high pedal forces which will overcome the tiny spring in the master cylinder many times over.
Aside 2
Fast-fill stepped bore master cylinders are very different to the 240 stepped bore master cylinder. Fast-fill cylinders use one large piston to move a lot of fluid quickly to take up clearances in the brakes and then two smaller pistons to actually apply pressure and balance it between two circuits. So 3 pistons in total. The smaller piston(s) apply pressure and are the diameter that should be used for calculations. This type of master cylinder can be used for front/rear split systems.