Brake fluid weeping at brake light swtich. Fix with sealant? Teflon tape?

[Bob Prosser]

Hey all. I'm going through my '88 daily driver, doing a little servicing here and there. I could use some input on one little thing that's driving me nuts.



I had a hydraulic brake switch go out -- the rear one on the master cylinder. (Not the first time.) This time I decided to replace it with a NAPA #SL147 (cross references to Neihoff #WA627C; Borg Warner:#S551; Harley-Davidson #72023-51C).



I put it in and the electrics work fine. Problem is, after a day of use I am getting a slight weep of hydraulic fluid around the threaded connection. I've tried pulling it out, cleaning, putting back in; tightening it a little extra; and even tried Teflon tape (not recommended I know). After some use I find it still weeping. So I pulled that one out and installed another identical new switch. This one leaked too. Very odd.

First time I've ever seen this. Anyone else had a similar problem or an idea? Is there a sealant I can put on those threads that allows me to install with brake fluid on the receiving threads?

Thanks in advance.

-

[slodave]

Is there a chance you over-tightened and cracked the MC? This happened to me once and was only noticed after the MC was taken out for a closer look. Just a thought...

Dave

[911pcars]

If all is well physically, you can use Loctite PST. It's a white pipe thread sealant; comes in a small tube.


Sherwoo

[rnln]

I had same problem, not 100% sure the cause but here what I experienced. I replaced them 3 times. First time, no problem. Second time, I has the leak, not much but enough to wet my fingers when I touch under the switches. Third time, no leak (it has been 1+ month now). In my case, I think it depends on the switch, not the master cylinder. Maybe not all of them are threaded the same way?

BTW, I have been through multiple switches, each pair only last around 2 months or so. After that, it starts slowing down (brake light come on too slow after the pedal is depressed). A while back, Draco introduced me his Arc Suppressor kit; which he has it on his car for quite a while. After looking at the circuit design and testing his brake lights, I installed the kit with another set of new switches. They have been working well on my car for over a month now. I will post the result after another several months. Here is the link if you want to read about the kit.
Brake Lights

[Bob Prosser]

Thanks for the input gents.

slodave: yeah, a hairline crack is always a worry, but I'm not new at turning a wrench. That rear unit is a b*tch to reach, so I used a universal on a 3/8" drive and socket with moderate torque. For the moment, I am not going to dwell on a possible crack (if I do I'll get a stomach ache).

911pcars: can I use Loctite PST in a "wet" condition? Meaning, I can have dry threads on the switch, but the receiving end will be wet with brake fluid. Will that bond? (FYI, the last OE unit that I took out had a red residue on the threads. No idea what it is.)

rnln: your experience is "encouraging" to read and is an exact explanation of my situation (I'm up to #2). I was even wondering if the leak is coming through the switch, meaning it's not hermetically sealed.

[911pcars]

"can I use Loctite PST in a "wet" condition? Meaning, I can have dry threads on the switch, but the receiving end will be wet with brake fluid. Will that bond? (FYI, the last OE unit that I took out had a red residue on the threads. No idea what it is.)
"

Bob,
Hmmm. Ideally, you'll want the mating surfaces to be clean. Try this. Before removing the leaking switch, depress the brake pedal about 1/2 way and mechanically hold it in this position with a prop rod or equivalent. This moves the MC piston halfway down the MC cylinder and blocks the fluid path between the reservoir and the cylinder. At that point, you can remove the brake switch. Residual pressure in the lines willl cause a slight loss of fluid, but after that, no more fluid will be under pressure (pedal or gravity).

Use cotton swabs soaked in brake cleaner until the threads are clean and dry, then reinstall/install the new switch with PST on the threads. Don't apply too much. I'd let this set up for a couple of hours before removing the prop rod.

Hope that helps,
Sherwood

[3.2 CAB]

Bob, when you used the teflon tape, did you have just a single wrap or so on the threads, or was it wrapped at least 3-4 rounds of tape? I do not believe that you will have any problem if you want to use the tape, as long as you DO NOT, have any of the tape that extends beyond the threaded area, onto the flat or tapered area where the actual hole is in the switch. If you keep the tape a couple of mm's or about 1/8" from the actual end of the threads, there should not be any worry about any tape pieces getting in your brake system internals. Good luck!! Tony.

[rnln]

"leaking through the switch"
You mean internal of the switch? Maybe, I never thought about it but it makes sense. On the third set, I thread it to the torque spec, maybe 16 ft/lb or so, and it stop leaking. I think I did to much on the second set.

On the brake light, it comes to the point that I was going to mount an LED on my third brke light for me to monitor when the brake light come on. After installed the Arc Suppressor, I have that "feature" automatically. The relay clicking sound tells me when the light comes on
If you are anywhere near OC, you are welcome to check it out.
Good luck.

Originally Posted by Bob Prosser

Thanks for the input gents.

slodave: yeah, a hairline crack is always a worry, but I'm not new at turning a wrench. That rear unit is a b*tch to reach, so I used a universal on a 3/8" drive and socket with moderate torque. For the moment, I am not going to dwell on a possible crack (if I do I'll get a stomach ache).

911pcars: can I use Loctite PST in a "wet" condition? Meaning, I can have dry threads on the switch, but the receiving end will be wet with brake fluid. Will that bond? (FYI, the last OE unit that I took out had a red residue on the threads. No idea what it is.)

rnln: your experience is "encouraging" to read and is an exact explanation of my situation (I'm up to #2). I was even wondering if the leak is coming through the switch, meaning it's not hermetically sealed.

FYI, I've replaced four switches in all the decades/miles I have owned this car. It gets to the point that I do a regular check in the reflection behind me (chrome bumper at a stoplight or office window in a parking lot).

[slodave]

I hope it is not a cracked MC. BTW, it was an experienced mechanic that replaced the MC a few years ago. There is no proof that he actually over tightened. It could have come out of the box that way. I just wanted to let you know. It was quite a surprise when my 911 decided to almost make friends with the VW in front of me.

[Por_sha911]

Did it leak when you put in the "crap" stock replacement? If not, then maybe you need to go back to what fits?

[Bob Prosser]

Guys, I took some time off today to get to the bottom of this. I did a leak test, using 60 psi of air, with the part under water. The stock unit (which no longer works as a switch) did not leak, but the replacement part leaked air. Conclusion: the threads were bad right out of the box. Their machining is bad. That explains it.

I installed a part from a different vendor, and the problem was solved.

But I am installing a "normally closed" mechanical brake switch at the pedal as well.

Life with a classic car...

[911pcars]

"..But this only buys me time so screw this -- I am installing a "normally closed" mechanical brake switch at the pedal."

Hooray for you Bob. The MC brake switches are a poor solution. If they're so unreliable, maybe the cruise control circuit shouldn't be using these same switches to override the system.

Sherwood

[rnln]

ah... that proofs my guess was correct, bad thread
MC can go bad too. The MC on my BMW went bad twice in 3 years. Well, it was a famous problem for the E36 model.

Originally Posted by Bob Prosser

Guys, I took some time off today to get to the bottom of this. I did a leak test, using 60 psi of air, with the part under water. The stock unit (which no longer works as a switch) did not leak, but the replacement part leaked air. Conclusion: the threads were bad right out of the box. Their machining is bad. Crap parts, I guess!

Dealer is B/O on the parts. I went to a VW specialty shop and purchased two from them (they look the same -- not sure if they are the same internally).

But this only buys me time so screw this -- I am installing a "normally closed" mechanical brake switch at the pedal.

[rnln]

I think it has been 4 months now, after I installed the Acr Suppressor on my car. My brake lights work fine now, just like the day it's installed.