We went for a Tilton in this instance, but the AP version does exactly the same job.
It took me a long time to understand exactly what a prop valve does. They often seem to be termed “bias” valves, and to an extent they do change the front/rear distribution of brake pressure (at the calipers), but they only do this once a certain rear brake pressure (generated within the MC and then transmitted, unaltered,to the calipers) has been achieved. But then, depending on the position of the prop valve lever, once a particular rear brake pressure has been achieved, any further increase in pressure (at the MC) will only partially be transmitted to the calipers. With both the Tilton and the AP valves, I think it’s reckoned that it increases at around half the rate of increase prior to reaching the threshold set by the lever position.
In another car that I have an association with we have brake pressure sensors in both front and rear circuits that measure actual pressures reaching the calipers. From observing these pressures we could see at what pressure the rear brakes would be most likely to lock up. And then the prop valve lever could be set to position which matched that value.
In terms of numbers, we found we could achieve up to around 80bar of pressure in the fronts, and with no intervention from the prop valve, the rear pressures would reach around 60bar. But beyond around 45bar the rears would tend to lock up (but fronts were still ok at 80bar). So we set the prop valve to setting 3 (AP valve in this instance) and the 80:60 split became more like 80:40. Of course, those values could have been adjusted via a different pedal box balance bar position, or different pads or tyres etc etc, but driver feedback suggested that the way we dealt with the rear lockup situation, in this instance, was to their liking, so we’ve stuck with it.
Back to the MX5, and prior to removing the servo, we actually found that front lockup was our biggest challenge. But at this time we had already elected to use really low friction pads in the rear. Various front pads were tried, including XP8s, but we ended up with Yellow Stuff, and they seemed ok, although shortly after we went to the servo delete delete setup and from AP calipers - which I don’t recommend lol.