Need some advice from the brake pros

bodde911sc · 10-29-2018, 03:38 PM

Looking for some help with and issue on my 80 sc. I just replaced the master cylinder with a new 23mm ATE unit and finished up bleeding everything this weekend. Peddle is firm and everything feels good, but upon turning on the ignition switch the "brake warning light" is on and the brake lights are crazy! The lights are on with no pressure on the peddle and as the brake peddle is pushed about 1/2 way down the lights go out for a split second, then as the peddle is pushed further the lights come back on! Weird, as before the replacement MC was installed everything was working well.
Wondering were to begin? switches?
Looking for some help.

juanbenae · juanbenae's Garage

id look closer at all the wiring leads to the new MC. also could be a compromised vac assist line that is causing a low enough pressure to light the light, yet not enough to effect normal, light braking efforts.

im full of bleep most of the time, so wait and hope mr verburg has not gone to bed already and chimes in. his insight would even be worth waiting till morning to enjoy & digest over a cup of coffee....

Quicksilver · 10-29-2018, 06:08 PM

Are you sure that the pushrod to the cylinder has freeplay when the pedal is released?

The piston MUST come all the way back to the retention ring/clip in the bore for the system pressure to be released. (It could be the piston sticking in the bore too.)

- This wouldn't explain the tail lights going off partway through the stroke. (Also your description isn't clear when the warning light and the brake lights are "going weird". Details are important) The brake lights are the more important part of the problem.
I wouldn't be to worried about the warning light initially. There are procedures to reset the front-rear warning switches.

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by bodde911sc

Looking for some help with and issue on my 80 sc. I just replaced the master cylinder with a new 23mm ATE unit and finished up bleeding everything this weekend. Peddle is firm and everything feels good, but upon turning on the ignition switch the "brake warning light" is on and the brake lights are crazy! The lights are on with no pressure on the peddle and as the brake peddle is pushed about 1/2 way down the lights go out for a split second, then as the peddle is pushed further the lights come back on! Weird, as before the replacement MC was installed everything was working well.
Wondering were to begin? switches?
Looking for some help.

assuming you retained he 3 prong switch on the side of the m/c, the center post is 81a, the other 2 are 82a and 81, one 81 is for the front circuit the other for the rear internally there are 4 posts, 2 for 82a ana 2 for 81a

there are 4 possible states

1) off when both external 81 posts connect to both internal 81a posts
2) stop when both external 81 posts connect to both internal 82a posts
3 & 4) failure of either the front circuit or rear circuit, here one of the external 81 connects to an internal 81a and the other to internal 82a

bodde911sc · 10-30-2018, 05:08 AM

Bill,
Master cylinder did retain both 3 prong switches for both circuits. Could this be a failure of bad switches? Or could have I switched the connectors from front to back? Not sure if that would make much of a difference. Is there away to incorrectly connect the electrical connectors?
Have any ideas on where to start?
Thanks for your input!
Tom

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by bodde911sc

Bill,
Master cylinder did retain both 3 prong switches for both circuits. Could this be a failure of bad switches? Or could have I switched the connectors from front to back? Not sure if that would make much of a difference. Is there away to incorrectly connect the electrical connectors?
Have any ideas on where to start?
Thanks for your input!
Tom

It's possible that the switch is bad but it doesn't matter which of the side poles is which.

The suggestion to be sure that there is pedal play(10mm is the spec) is a good one.

Pull the pedal back by hand and adjust.

bodde911sc · 10-30-2018, 06:05 AM

Ok, will check pedal free play and see if pressure is being released.
Any thoughts on why brake lights are going out for a moment when pedal
is about halfway depressed?

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by bodde911sc

Ok, will check pedal free play and see if pressure is being released.
Any thoughts on why brake lights are going out for a moment when pedal
is about halfway depressed?

In the m/c there are 2 floating rods which hold/position the seals, one for the front and one for the rear, there is a spring between them, so the stroke of the primary is longer than the stroke of the secondary. The actuation is through the primary which compresses the spring to actuate the secondary

The switch is hydraulically actuated, in normal use the fluid pressure pushes the internal contacts in the same direction to connect both external 81 posts to both internal 81a posts, when in the off position, and 2 the 2 internal 82a posts in the brake light on position. The failure mode is when only one of the internal posts is connected normally, either on or of and the other is in the opposite state.

In yours there may be a temporary lag in the change of state of at least one of the internal switches.

usually when the switch goes bad the internal switches need more and more pressure to change state, causing a larger and larger lag but in in both circuits at the same time

KTL · 10-30-2018, 01:54 PM

I'm going by memory here from a number of years ago, relating a problem we observed on a local friend's '87 Carrera. We were at the local track and his hokey, dim 911R style LED lights (this is when these LED tail light units were in their infancy) were staying lit. So when he was on the track, nobody knew when he was on the brakes. Turned out to be one of the switches on the MC and I believe we just disconnected one of them and the brake lights worked for the time being.

These switches do fail over time and their failure modes seem to vary. They can take a while to activate, which means you must press the pedal harder than normally expected to turn on the brake lights. I had this issue on my car where I can sit at a stop light and have my foot on the brake pedal, lightly depressed, and I see no brake lights reflecting off the car behind me. Push a bit harder and they come on. Some don't come on at all. Some stay on...... Weird

Bill Verburg · Bill Verburg's Garage

If you have dual switches, the center pole is a dummy, only the 2 outer count, 1 switch fir the front and the other for the rear

bodde911sc · 10-31-2018, 06:43 AM

Bill,
is there the possibility that I've got air trapped at the pressure switches? Have you ever had that happen? I didn't bench bleed the new master cylinder before install, but I've never done that before and never had and issue like this. If there is air trapped at the switches could I pressurize the reservoir and just lightly crack open the switches (if I can get a wrench in there!)
and bleed out any trapped air?

KTL · 10-31-2018, 07:21 AM

Quote:

Originally Posted by Bill Verburg

If you have dual switches, the center pole is a dummy, only the 2 outer count, 1 switch fir the front and the other for the rear

Thanks for clarifying Bill. Now that I think about it more, I believe what we ended up doing to get the brake lights functioning again was to swap locations of the harness connections. Then of course installed a new switch later.

Air doesn't seem to gather around the switches. Typical pressure bleeding does a good job of pushing all the air out even if the MC hasn't been bench bled before installation.

bodde911sc · 11-05-2018, 07:14 AM

Weekend update; checked brake pedal freeplay and everything was good there. Decided to go back thru master cylinder and check everything over before removal. Removed both electrical connectors from MC and checked that warning light went out and no brake lights were working.
Next I installed just the front connector and checked lights. Interior warning light was out and brake lights were working! Now I removed front connector and reconnected back connector, upon it's removal I noticed that one of the connector tabs on the switch was "bent" down, I straighten it out and reconnected. I believe it was the center tab. Once connected I checked warning light and brake lights and everything worked as normal. Reconnected front connector and everything is working perfect.
I thank everyone here for there comments and suggestions!