Today I rebuilt my LR brake caliper and fully bled the line to the caliper on my sweet '81 928.

Initally all was well.

But quickly the "!" warning lights and "Brake Press" warning began flashing.

The brakes work fine with no softness, and the fluid reservoir is full.

Can anyone tell me why this is happening and how I can get the warning flashing to stop? I drove car about 10 miles, testing brakes, turning engine off and on and warning light continue to flash.

Is there a way to reset the brake system?

TIA,

GN

 

how old is the master cylinder?
if its older than 20 years its probably time to replace it.
NOTE it will come with new signal switches.

That said you could try swapping out the switches that are screwed into the bottom of the MC now .
HOWEVER
My guess is that since you bled the system the pedal went full stroke and the MC seals got damaged,
this is now sending a mismatched signal to the switches that are working as they should.

If you swap out the MC get an ATE replacement,
use ATE Gold brake fluid ,
I suggest to use Molycoat on the MC grommet seals where they seat into the MC ,
this will prevent the inlet bores from rusting.
NOTE when the car gets wet ,
water will rinse down the tank and flow over the inlet bores of your new MC ,
with no Molycoat the water will seep into the rubber grommets and rust the inlet bores.

Clean the fluid tank with some brake cleaner first,
then use simple green and hot water rinses,
followed by pouring about 1 ounce of alcohol into the tank,
then shake for 20 seconds to remove the water ,
then drain it,
then blow dry with a hair drier.
Put some Molycoat on the inlet barbs to ease installation.
Consider replacing the 4 brake lines if they are still l the rubber versions with the new stainless steel flex lines

 

You can disconnect the battery to try and reset the pressure switch warning - that's the only thing that Bill do it once activated. If it comes back on you'll need to replace the switches on the underside of the master cylinder and most likely the master cylinder Any chance you got them wet, like spilled brake fluid at the reservoir and decided to wash it off with water? That will do it too as the protective rubber boots over the switches act like cups for water and the contacts are on the underside in the "cups". If so, slide the rubber boots down, clean and dry the sensors, let them fully dry, then do the battery disconnect and see if it resets

 

Mrmerlin -

Thank you so much for this info - but what a nightmare! A simple caliper rebuild turns into a major project!

Was there a way to bleed without the pedal going full stroke?

No idea if MC was ever replaced, multiple owners before me but last two did a lot of replacement stuff so car was mint when I got it. I would expect it has been replaced but have no way of knowing.

GN

 

Thank you Petza -

Hoping and praying the battery disconnect fixes this. Need new battery anyway.

No way the underside of MC got wet in last 2 years I've owned the car.

GN

 

Do you have a pressure bleeder for this kind of work? I bought a Motive for it and have never needed to move the brake pedal during flushes or after other work replacing brake components. There's a couple of ways to use them (fill the Motive reservoir with fluid, or just use it as a pressure vessel and refill the car's brake fluid reservoir frequently) and they both work well.

Good luck

 

I have a good friends pressure bleeder and was going to use it. While the cap fits 944's, the 928 has a much smaller cap and we could not use it.

I'm going to buy a 928 and hope to find a fitting tor it to use it with the pressure bleeder.

GN

 

Found the master brake cylinder was replaced by the PO. Good news. Hopefully disconnecting battery will reset the computer.

GN

 

Bled both front & rear brakes with pressure bleeder, disconnected battery for 10 min., all brake alarms GONE!

HALLELUJAH!

Thanks for all the help & advice here.

Now have new electrical problem....short in circuit for right marker lights and taillight.....cannot find it.....

GN

 

glad you reset the brake warning.
Can you be more specific about the RR light?
whats happending?

 

Right marker lights and right rear tail light circuit blew fuse - has short in it. Cannot find short.

I am an EE of vast experience, 80 years old. Think my mind is over 95% there. I was also an electrical contractor and master electrician at one time many eons ago.

To locate a short, you isolate different parts of a circuit and test each individually. On the 928, the wiring is even more concealed than in most cars, and I cannot isolate the circuit into its different parts. Found no short in rear tail light socket so far. Have not pulled right rear marker light but will.

I tried a new fuse and saw smoke (had to be from short) in right front area where fog & driving lights are. See no problems there so far. Connectors are there behind the lights for the right front marker light.

Any help finding this short is greatly appreciated!

GN

 

OK first thing disconnect the 2 pin connector on the hatch floor RR corner,
this is the RR side marker power , see if anything changes,
if so then remove the RR side marker holder from the lamp housing and inspect for touching contacts.
If no joy remove the tag lamp screws and pull the housings out inspect for shorted contacts
NOTE its possible that removal of these tag lamp housings could require them to be replaced .

If no change then remove the RR hatch floor bracket tiedown ,
NOTE removal of the bracket will prevent damage to the lower liner edge.
spray PB blaster at the underside of the chassis to allow the screws to come free.
Once the bracket is removed then 2 more screws for the aft edge of the quarter liner.
get a small piece of 2X4 and gently pry the aft edge of the liner forward,
put the wood on the floor to hold the liner away from it seated position,
fish out the RR lamp barrel connector,
In the barrel Pay attention to the 2 brown wires that are crimped together these come loose or can short .
I usually resolder them.

If no joy, disconnect the connector, then retest to see if the fuse still blows
if so then follow the wires to the CE panel plug on the bottom of the CE panel look for a single connector that could have pushed out of the colored one.
Your year is susceptible to this anomaly .

 

MrMerlin - WOW! Amazing detail!

Never knew RR hatch floor could be opened. Will try to implement tomorrow.

"Your year is susceptible to this anomaly" If so, I would think there would be one single most likely location for the short. But as you did not mention one, but several locations, that must not be true.

Cannot thank you enough.

Your level/depth of such facts is remarkable to me - an experienced mechanic and electrical guy. How did you accumulate this deep and detailed knowledge?

GN

 

Oh, I pulled the front right marker unit today and there is no short there. I disconnected it from the connector behind the fog & driving lights - short remained.

GN

 

thats not where I told you look????