Another Question, Brake lights this time

[JoeF]

Brake lights wont work, I have checked the easy fixes, ( fuses, Bulbs, circuit to master cyl.) but I cant seem to fix it. The car is a 81 930, I went looking for a brake light switch under the floor board behind the pedal cluster, but it wasnt there. The car has a dual master cylinder and what appears to be two sending switches pluged in, there is power to each one. Can any one give me a hint?

[JDaniel]

Do the lights not work period, or do they only come on when you really push hard on the brake pedal? If it's the later option, then check the archives. It could be faulty brake switches. These are located on the M.C., not on the pedal itself. It's a cheap and easy fix.

[JoeF]

Dan,
Wow! You are good!!!
I marched out there, steped on the brakes hard, the brake lights lit up! Also, now they appear to be working normally!
Dont worry, I will still change the switch anyway.
Thanks alot!

[Nifty]

I had the same problem and eventually traced this to the 2 switches on the master cylinder

however is it really that easy to replace these, won't you need to drain the brake fluid first, given the aggresive nature of brake fluid I don't want to spill this inside my car

[Jdub]

The secret to replacing these items (at least on my SC) is to go ahead and start loosening them in the trunk, then have someone provide positive pressure to the pedal (VERY slowly) as you quickly remove one and replace it with the next one.

Not tough to do. Get everything out of the way: hoses, cruise control module, etc. for clear access. Loosen both switches with your deep socket or bend the three (or two) poles inward to provide socket room internally. When switches are at hand loosening state, have helper SLOWLY press on pedal, NEVER removing pressure but keeping it steady. Remove the one switch, have helper push pedal down to exit all the crap you will find there (i.e. at the "bottom" of the master cylinder; basically a purge) then quickly screw in the new item. Repeat for the other one.

On purchasing the new items. Many insist that the VW-grade switches will work fine. I am still out on this one: my new VW replacements lasted all of five months. You might want to spring for not only real Porsche switches, but see if you can poke around the box they keep all of them in at the dealership to find two that come from the same manufacturing lot. Much like buying two or more spray cans of paint, you always look at the bottom of the can to find the lot code and make sure you buy from the same lot number so that you know the colors between cans will match. For the switches, since they work on pressure, you want to know that they were built and inspected at the same time so that the pressures between the two will be matched at the factory. This prevents the irritating brake dash light problem wherein the differential in pressures caused strictly by the switches (all other things aside) causes the light to go on. Better-matched switches can obviate this problem.

Jw

[Jdub]

Just revisiting this thread...my switches are now out once again.

This surely is one weak spot in our cars. I bleed my brakes on a regular basis and other than using the car on a regular basis there is no reason for these to fail other than bad design/bad manufacture.

Can anyone tell me if the Porsche versions of these switches (I used VW) are ANY more reliable? These bits don't last very long at all (just like the horseshoe/helper springs!).

John

[Early_S_Man]

John,

How about Mercedes switches ... at least you know they were designed for a system with a vacuum booster??? I have my doubts that the Ate switches really have a reliability problem. Normally the problem is contamination from rubber particles (sludge) plugging up the pressure ports.

Switch reliability aside ... have you considered that your forward circuit in the master cylinder (nearest the booster) may be leaking pressure due to worn seals and cups?

[Jdub]

Warren:

Thanks for the reply - I have studied your posts on this subject and do agree that degraded rubber and moisture can kill these switches before their time.

The forward circuit may very well be decomposing on me. Having rebuilt several master cylinders in MGs and Triumphs I do remember the awful internal state of what seemed to be a nice unit from the outside. No rest for the wicked.

Two questions:
- Once "plugged," can I return a switch to near-new conditions by cleaning it (assume a non-caustic soak and blast of air). I guess they ARE cheap enough...maybe just buy new and be done with it?
- Can a Porsche master cylinder be rebuilt, or is the best (read: safest) buy a new unit?

Many thanks,
John

[Early_S_Man]

John,

Yes, I have cleaned many of the Ate brake switches by using Brakleen and Q-tips, followed by compressed air ... never had any problem if there is no evidence of rust or corrosion at the switch pressure port.

I have sucessfully rebuilt VW and Porsche non-boosted, and BMW & Mercedes boosted master cylinders with Raybestos-boxed Ate rebuild kits. That said, I have heard that there is no available Porsche rebuild kit for the SC master cylinders! I haven't checked for a Raybestos kit avilability for SC's. But, if there is only seal/cup deterioration and no bore pitting (passes the 'fingernail' drag test) ... I see no safety issue with rebuilding the Porsche boosted master cylinder, personally. Liability lawyers included in the equation, that goes against most shops stated policy these days ... but they aren't working in an ideal world any longer!

CYA ... 'Bentley-style,' is the byword these days!

I have never had a problem with a rebuilt master cylinder, using Dow Corning 4 silicone grease on the cups and seals ... in over 30 years!

[Jdub]

Here is what I have found using Raybesto's web site:

Raybestos SC/Carrera master cylinder rebuild kits:

For SC/Carrera, rebuild kit:
MK670

For SC/Carrera, master cylinder new:
MC39292

I am looking into purchasing one now and will get back to the group.

Many thanks,
John