DeTomaso Mailing List: April 99, Message #126

Chuck,

You wrote:

>OK.  The brake actually holds the car on an incline, but the problem is that 
the parking brake light won't go off.  It stays on all the time. 

>>>Remember, that light is a dual-function light--one of its inputs is from 
the parking brake handle, the other is from the brake pressure low pressure 
switch.

Your mechanic has been horsing around with your brakes, and I presume he 
wound up opening the rear circuit to manipulate various things.  When he 
stepped on the brake pedal to bring back pressure back there, the junction 
block sensed the differential pressure, interpreted that (mechanically) as a 
ruptured brake line in the rear system, the sliding chingus slid as it's 
supposed to (technically, it's a hydraulic fuse), blocking any further fluid 
from flowing to that system, and now you have NO rear brakes, and an idiot 
light glaring in your face.

To test the theory, open the front trunk compartment.  Find the brass 
junction block bolted to the floor of the trunk (right below the master 
cylinder.)  There's two pressure switches there--the larger one has two wires 
and is your brake light switch; the smaller one is your warning switch.  
Unplug that wire, voila, the light should go out.

Of course, you still have no rear brakes.

Getting that sucker to reset is (according to Jack) one of the greater pains 
in the butt around.  In fact, just a few weeks ago on my car I trashed this 
entire system and replaced it with normal-normal brass junctions.  The stock 
setup makes perfect sense if you're powering all four corners with a 
single-chambered master (you want a means to isolate the front from the rear 
in the event of a failure, so you still have brakes), but when you've got a 
dual-chamber master, you essentially have dual independent systems.  The 
sliding hydraulic fuse in the junction block just gets in the way...

If my diagnosis is correct, Jack might be able to tell you a better way to 
fix it, but it's possible that you'd have to remove the damn thing and use 
compressed air to get it to un-stick itself from its seated position.

If I took one of those off a car that had high-performance aftermarket dual 
chamber master on board, I'd never put it back again...

Mike (due for a brake bleed now!)