[gva-sfo]

Yes, great topic.
I have this problem, ..but I think from another reason : being a lover of *firm* brake pedal feel, I changed the OEM master cylinder (of my 2001 Boxster S, that has PSM) against a TRW (oem supplier) larger one. (used on GT2 and GT3).
The results are amazing, just getting the perfect brakes !
(it is true that in the front, I have 350mm floating rotors and 6 piston monobloc calipers).
Well, since the change of the master cylinder, I have a lot of "4460" with ABS And PSM lights on. Some time I'm free of these for 100 miles or so, hoping for cure, but... no, lights come back !

There are plenty of theory on that, where the big MC do not have the same fluid circuit than the OEM MC.
This may be one of the reason, i.e.: building up too much fluid pressure ..too early.

I want to revisit the switch problem (for sure, I did make that change).
But the one that I got, do not have the plunger "clicks" adjustment.
I still tend to think that yes, the problem is having too much pressure too early (btw, this is what makes the super great feeling of the brakes), but I think that what generates the PSM and ABS lights on (error 4460) is the difference on the timing between the reading of the pressure sensor (on the PSM pump assembly) ..and the Brake stop switch !
I.e.: if in the brake pedal travel, you have a certain level of pressure to close, ..or earlier than the brake on switch, .. then you got it.
I think one way is to have one of those brake pedal switch that has the clicks adjustment, or, may be a kind of Teflon spacer between to top of the switch and the pedal space that touch the switch, this to have the brake switch on a bit earlier.
I did not had time to work on this yet, but I'm going to try.
Some guys on other forums think that the big MC built too much pressure, but frankly, I do not think so. I would imagine that one that brake REAL hard, with the OEM MC of the 986, would also got high pressure, ..but much "later" in the pedal travel distance !

It seems to me that what I'm reading here, tend to keep me think that you really need to have Pedal brake on first, ..and later in the pedal travel distance, pressure is building up. If the reverse happen, I get the feeling that this does trigger PSM/ABS error.
Sure, if you limit the pressure (adding a pressure limiter in the circuit), you delay the threshold of the PSM pressure sensor, ..and solve the problem.
But, I'm for trying to act at the "other end" !!

Or, depending on our brakes, ie.: great brake fluid, very well flushed with good thick pads (and rotors !), I would think that a brake switch that is not well adjusted, and tend to go on a bit late, ..can be the real source of the problem.
More to come !
..and thanks again to all of you for the great topic.