Problem with 2-pole brake warning switch in 914R MC, Solved - I think... Options

[bbrock]

Long story short, I had the phantom relay clicking from the day I put my car together. I traced it to the brake warning switch on my 914R master cylinder. I did make sure the reset button was pushed in but the relay still constantly clicked without lighting the warning light unless I disconnected the switch.

I bought a genuine Porsche 2-pole switch and finally got around to installing it, but now the warning light flashes all the time. The switch works - closing contacts when the plunger is depressed about 1mm. The problem is that screwing the switch fully into the MC depresses the plunger enough to close the contacts and activate the warning light. Backing the switch out about a turn is enough to open the contacts again.

I'd like to understand how the switch plunger is actuated under low pressure condition to make sure I don't defeat the warning system with the wrong "fix." I can think of two possible cause/fix scenarios:

Best Case - Need a thicker O-ring. The 914R MC/switch did not include an O-ring so I grabbed what I had on hand that fit the diameter of the switch base, but is rather thin. I don't know the correct size for the OE O-ring, but the PET has a note for the part that reads "12,42 - 1,78" If that means 12.42mm I.D. x 1.78mm thickness, then that would likely space the switch back far enough that the plunger isn't depressed. Easy peazy!

Worst Case - looking inside the switch port I can see a small bore in the center about the diameter of the switch plunger. I have a sinking feeling that the plunger is suppose to fit inside this bore. If so, it means the bore in the MC is too small and the warning system is not functional.

Either scenario might also explain the phantom relay issue because it is possible pressing the reset pushed the plunger in far enough to shut off the warning light, but not far enough to completely open the switch contact and allow enough current to leak to ground to run the relay constantly. The 2-pole switch has the self resetting spring loaded plunger when might not go in as far as pressing the single-pole reset.

I'm really hoping it is the "best case" scenario. Otherwise, it raises the question if the warning system is working properly on any 914R MCs. Has anyone actually tested the low pressure warning on these MCs by having an assistant crack open a bleeder while the brake pedal is pushed and ignition on per the FM procedure? If "worst case," I'll have to bite the bullet and do what I should have in the first place - buy an ATE MC. (IMG:style_emoticons/default/dry.gif)

Any thoughts?

[bbrock]

Think I have this licked and all is working as it should. Did another bleed on the brakes this morning (about 10 psi with the Motive power bleeder). Did see much air. Just one bubble about the size of bird shot from the RR and another bubble a little larger than a BB from the LF. I didn't think it was enough to make a difference, but when I checked the switch plunger hole depth in the MC I got 20mm and couldn't tell for sure, but think I was seeing a fine line where the two balance pistons meet through the hole. Then I screwed in the switch and checked resistance. Read open circuit just like it should. (IMG:style_emoticons/default/piratenanner.gif) Connected the wires and turned on the ignition switch. No flashing warning light. (IMG:style_emoticons/default/piratenanner.gif) (IMG:style_emoticons/default/piratenanner.gif)

Drove the car about 40 miles giving it heck through mountain twisties and everything seems to be working properly. I'm surprised that small amount of air was enough to trip the warning but I could feel the difference at the pedal. Not much, but enough to be noticeable.

Anyway, another confusing troubleshoot with a confirmed bad switch on top of a slightly off balance system. Happy to have things functioning again.