There is another test you can do. Make 1 or more hard stops, quickly jump out and measure the temperature of the rotors. You can get a fairly accurate reading using a IR laser thermometer pointed through a wheel opening at the rotor face.
This will give you a quantifiable measurement (in heat energy) of the amount of work being done by the brakes on an individual basis. From that you can get a general idea if one or more are not performing as expected.

 

Good idea! will at least let you discover if they are truly doing virtually nothing under real world conditions. Even when they are working perfectly a 33bar bias valve definitely helps in better straight line breaking (needs a little more care at other times).

Alan

 

Single channel ABS to the rear was basically useless. Porsche added an 18 bar limiting valve to make sure the rear brakes would not lock up in any situation, especially in rain or snow.

Along with an almost useless (for any type of performance use) limited slip and a suspension that guaranteed understeer, 928's were fairly idiot proof to drive.

 

1. Removed driver's side rear caliper
2. Engine running, applied pressure on pedal
3. Only one of four pistons (outboard, lower) in the caliper moved
4. Brake fluid leaked from that cylinder

So, (and thank you all again), I would appreciate confirmation that what I observed is abnormal and a caliper rebuild is required. Asking because it seems obvious but I want to make sure there isn't some sequential pattern to how the Brembos are valved (like until one piston feels some initial resistance there's no pressure applied to the other four).

Also, the pedal (with engine on) was mushy and went to the firewall. All other rotors and calipers were still in place. Is that normal? And thus would that mean if I lost pressure from a leak at one rotor none of the brakes will bite? I thought (again not a mechanic so thanks all for your patience) the multi-chamber/piston configuration of master cylinders existed to provide braking if one caliper failed.

 

Not an ideal test. But the leak is concerning and expected if you pushed the piston far out of the piston bore. This test would simply identify which piston has the lowest seal drag or threshold pressure. It doesn't suggest your calipers are bad but it doesn't suggest they are good either. The way I do this test is to substitute the rotor for a block of soft wood slightly thinner that the rotor and note how much difference in pressure is required to get all the pistons to move. It's usually just a few PSI but the test you did allows the "free-est" one to move thus allowing no addition threshold pressure to go to the rest. Off the car I do this on the bench with air pressure, easy to observe. On the car you'll need a camera to observe movement and will only be able to guestimate threshold pressure. Replacement calipers may or may not be the answer in your case, I've seen "rebuilt" multi piston caliper have 1 or 2 complete stuck pistons right out of the box so if you are going with new calipers choose your supplier carefully.

Yes with a caliper operating with no rotor will make the pedal travel all the way to the floor. This is a risky endeavor becasue this allows the Master Cylinder piston to travel far or deep into the bore where it usually doesn't go so there could be corrosion that can destroy the MC piston seals. It's very common for DIY'er s to replace their calipers and bleed them with a helper using the pump and dump method only to find the brakes feel worse becasue now a new M.C. is needed. Experienced DIY'ers knows a 50$ pressure or vacuum bleeder and frequent fluid changes keep brakes running right and tight.

In any case this test is mostly unnecessary, the piston should move in their bores with moderate finger pressure when the caliper is unconnected or a bleeder is open. On older 4 piston calipers like the ones that Porsche on the 928 uses the manufacturing tolerances are not as good as they are on new calipers so pressure required for movement can vary a bit. For example on new Wildwood and Audi mutipiston calipers I've worked with the pistons will all move in concert in free air like the test you did. I presume this is becasue precision manufacturing allows for a more exact threshold for piston movement. The older multi Piston Porsche and Jaguar multi piston caliper I've worked with are not like that, each piston wants to do it's own thing or has a different threshold pressure for movement. That said my Porsche brakes still feel FAR FAR better than my Audi RS6 brakes but I suspect that has more to do with line length or over all weight bias of the car and not a few PSI of brake piston threshold pressure.

Good luck, lots of physic and material science to consider with brake systems. If you're not up for that consider a local / trusted pro.