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.

Any thoughts?

Get the ATE master.

Zach

I'm interested in this too. Maybe you could use a thicker crush washer instead of the o-ring?

I have developed an issue where my brake warning light stays on. 3/4 of the time the car is on. Upon inspection I do not see any leaks ( a very slight leak at the rear caliper adjuster but...) so its hard to imagine any other cause than the switch itself. Not to derail your thread but- when you replaced the switch did you need to bleed the whole system again? Or can one just pull it out and throw in a new one and be good to go? Mine should be good as it is low miles on a 21mm ATE master.

Pete

A crush washer was my first thought but without knowing how the switch is actuated, it would be a gamble. It would stop the warning light from flashing, but may be the equivalent of disconnecting the switch completely. Not something I'm willing to accept..

You do not have to bleed the system when replacing the switch (I had the same question some time ago). There should not be brake fluid in the switch port.

@Mikey914 As this is a potential safety issue, it would be good to know if the low pressure warning system has been tested on 914R MCs configured as shipped (single pole switch with no O-ring) using the FSM procedure. If so, that would quickly rule out the MC itself. Perhaps something as simple as installing the correct O-ring is all that's needed to position the switch for correct function. I'm pretty sure 12.42mm x 1.78mm is the correct O-ring size as that seems to be a common size. Bottom line is that I (we) need to be confident the warning system is working properly.

BTW, I've confirmed that the switched that came with my MC was defective. It reads ~5Mohms regardless of plunger position so never operated correctly and leaked enough current to ground for the phantom flasher clicking.

The plunger is supposed to contact a special piston inside the master cylinder side chamber. The piston sees pressure from both brake circuits and will move due to pressure differential. If the piston moves in either direction, the diameter at the plunger increases, causing the switch to close. The reset will only work once the piston has returned to the normal operating position.

At least that's how the factory MC works.

Very helpful. From your description, my guess is that the switch plunger is positioned between the two conical bits on the piston at the arrow in the below diagram. If correct, that leaves 3 possibilities for my problem:

1) plunger is not sliding into the small bore inside the side chamber.
2) the piston in my MC is not centered where it should be (brakes were power bled and pedal has held firm for 6K miles with no leaks).
3)switch is simply screwed in too far because of missing/incorrect size O-ring.

Click to view attachment

Isn't this at least the 2nd issue you've had with that master cylinder? If I recall it was a leaky / defective welch plug the 1st time?

ATE looking better and better - gotta' agree with Zach

We made ours to work with the switch that was commercially available at the time. They are not the same. Happy to throw you a replacement switch (single pole) if that's the issue. Next run we could look at using the OEM switches if they are available, but will take some re programing and testing.
Just let me know if you want one.
Mark

Well- it’s not the o ring sorry to say. Switch bottoms out metal to metal against the master cylinder body. I measured mine- 1.68mm Fwiw.

Pete

I’d measure the hole first using a drill bit as a go/no go. I suspect in my case that the rear caliper pistons may not be returning properly as I can not easily fit a feeler gauge in to check venting clearance at the pad without pushing the piston in a little. Not sure if this could be the cause or not. Also a hard push on the brakes turns off the warning light.

Anyway you might also measure the depth of the switch plunger bore to the switch mating surface with the same drill bit and compare it with the length of the corresponding portion of the switch.

Pete

Thanks Mark, I'm pretty certain the problem is not with the switch itself as the OE 2-pole and original single pole that came with the MC have the same plunger diameter and length. I did use a drill bit to check the hole bore and 9/64 bit which is slightly larger than the plunger diameter easily slides in, so that is not the problem. When I look down into the switch port, I can see a solid rod through the bore that looks to maybe be brass. I don't see anything that looks like I would expect from the diagram.

So if we've ruled out the switch, the O-ring (thanks for that), and the bore, what's left? Misalignment between the plunger and bore, or the guts of the balancing piston mechanism I suppose. I'd still like to know if these have actually been tested in the car as a complete system by cracking open a bleeder while someone presses the brake.

BTW, both versions of that switch are available through Porsche.

Here is the two-pole

and the single-pole

They aren't cheap, but worth it if they mean less time having to trouble shoot and swap out new parts.

Followed up on your idea to check the depth here Pete. Great suggestion. Looks like there is plenty of depth for the plunger to go in without be depressed so it is looking more like is just not findings its way down there. I'll try smearing a thin coat of brake assembly lube on it and see if that helps.

Holy smokes, that did it! Put just a dab of brake lube on the end of the plunger and kind of wiggled the switch back and forth as I screwed it in. Checked the resistance and get 0 ohms

I must have tried screwing that sucker in at least 5 times before with no luck. Must be just enough friction that to hold the spring in the plunger back (it is pretty weak) without a little help. More just one of those need to hold your mouth right things. Who'da thunk such a simple thing would create such a problem.

Next step will be to actually test to make sure the warning light actually comes on with low pressure in the circuits.

Nice! Glad that got worked out. I still need to trouble shoot mine....

Pete

Funny, but mine is doing the same thing. I have an ATE 19mm master cylinder. No leaks but the light came on. Bleed the brakes no change. Changed the two pole switch, light went out but relay clicks while I’m driving it. This has been gong on for over a year now. I thought it was the LED lights causing the problem. I will have to try lubing the switch.

Well, my celebration was premature! Went out to button up a couple things on the wiring such as install waterproof sleeve and swap out the temporary terminals I crimped on for proper flag terminals. When I pulled the boot off the switch, I found one of the temporary terminals had popped off the switch. Didn't think much of it since I know for a fact the switch was not triggering before sliding that boot on. I completed my tasks which took much longer than it should have of course. Connected the switch and dammit, the warning lights were flashing again.

Pulled the switch and tested it again. It's fine so the problem has to be in the MC. I measured the depth of the hole for the switch plunger at 17mm and compared that with my original MC that this one replaced. It is 20mm. The difference is about the difference between what it should be with the balancing pistons centered and with the system tripped. Either the balancing pistons in my MC are not centering properly or I have a pressure imbalance. If it's the latter, just how sensitive is this system? My brake pedal is firm and my brakes seem to be working quite well. I'll do another bleed anyway just to see.

In the meantime, I've started disassembling my old 17mm MC and am looking for a NOS rebuild kit. I'd rather have a 17mm MC anyway but not thrilled with the idea of having to remove and replace the MC yet again.

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. Connected the wires and turned on the ignition switch. No flashing warning light.

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.

Pretty sure this is what I need to do as well. I ordered the Motive and hopefully will get some time.

I do notice that the front brakes lock up and with my 21mm master I have to push pretty hard to get that to happen (911s calipers and 914-gt rears) but still no lock up in the rear. My light does not blink- its a solid on. Hoping that bleeding the rears will solve this issue.

Yes- any small bubble will have an effect- especially when the brakes get hot.

I'm not understanding the system well enough to know what it means that the light is always on but I do know that this was not the case before. I think some aggressive braking/stopping to test the brakes may have been when the light first lit up.

Pete

Pete

Check out this thread, and the resolution in my build thread.
http://www.914world.com/bbs2/index.php?sho...master+cylinder

So what was the resolution? I couldn't find how you solved the issue in the thread.

Just wanted to follow up on this post. I ordered a new stock pressure switch from FVD and compared it to the one that was in my master:

Click to view attachment

Hard to see in such. poor picture but the new one is 1-2mm shorter than the one that cam in the master. I switched it out and the pressure light is no longer lit. Have not had any issues since then!

I kept the old one and will shorten it a bit and keep as a spare.

Pete