Master Cylinders
Switch is probably stuck on after bleeding the brakes
Go under the steering column , switch is on the pedal shaft. Disconnect one wire - if light goes out, replace the switch.

EDIT- sorry I screwed this one up
On the pedal is for the brake lights
Warning light is controlled by prop valve.
Don but is that switch there for a 69 mustang under the dash?
(09-19-2015, 11:15 PM)rindel Wrote: Don but is that switch there for a 69 mustang under the dash?

Pretty sure this is the switch Don is referring to:
(09-19-2015, 10:29 AM)rindel Wrote: Hey guys, I was trying to troubleshoot why the brake idiot light won't go out. I bled them a few times and the light would go out then come back on after a few pumps on the brakes. After a while my brakes would occasional sync and i had to do the double pump to stop the car. This would continue even after bleeding the whole system 4 times (rear passenger, driver, front passenger, driver). I ended up replacing the MC and my brakes are now working fine but the only thing left is the brake warning light. I bled the brakes again after change the MC but still stays on. Could this be proportioning valve or distribution block? someone with a 66 mustang told be there is a switch behind the brake pedal. Never heard of it before.

The switch attached to the brake pedal under the dash is for your brake lights, not the dash brake warning light. Something is going on with the pressure differential valve, or, check to see if your emergency brake is also wired to control the brake warning light. That was an optional feature in 1969.

Maybe the spool valve inside the pressure differential valve is not getting completely centered or sticking so it won't automatically center. I've never done it, but I've heard you can remove the electrical switch from the pressure differential valve and see the spool valve inside and move it with a small pick or something.

Maybe the article below will help. It's for Ford Cougars but the pressure differential valve and proportioning valve look similar. I think I would remove the pressure differential and proportioning valve and rebuild them. There are rebuild fits from NPD for the pressure differential valve and the proportioning valve. The kits are basically just new seals. NPD also has a special bolt to hold the differential valve in position while bleeding the brakes.

Note: If you decide to rebuild there is a special assembly lube for brake hydraulic parts that most parts stores carry.

The lube for brake parts is Castrol GRR (or similar but l have found that product to be the best I have tried).
The reset for that warning light valve is to remove the electrical part, pump the brakes and replace the switch. If it does not work it is a jammed switch or other hydraulic fault.
BTW the only way seals can be damaged in a master cylinder is if there is debris in the bottom of the bore that damages the seals as they rasp over.
A new m'cylinder will not have debris, so damage will not occur in a correctly adjusted system no matter how hard you pump
OK guys I have replaced the wheel cylinders and no leaks are present. Re-bled it all last night with a helper and my nifty new speed bleeders. Now my issue is that I only have brakes after I pump them a few times. I ran 3 pints of fluid thru the system. Its all clean and I am pretty confident it has no air. I'm stumped. I'm am using the correct bleeding procedure, new fluid from a metal container, now on my 2nd MC (new - no rebuilds), new wheel cylinders.

My father-in-law said it could be my booster but I have never seen a booster act like this. Usually it would just make the pedal hard to press. Can I still have air in the system?

Thanks for all the help guys and sorry to be a pain.
I don't know if you tinkered with the rear drum brakes, but are they adjusted correctly? The pedal feel will be a mess if they are not adjusted correctly.

Regarding rear drum brake adjustment, how is the operation of the emergency brake? Normal? If the E-brake goes to the floor it can either be the drum brakes are not adjusted correctly or the E-brake cable needs adjusting. For that situation the drum brake adjustment is checked first.

Does the brake pedal feel any different if the E-brake is applied when you push on the brake pedal? If so, the drum brakes are out of adjustment.

If the drum brakes are very far out of adjustment the normal auto adjusting during back-up stops can take a long time.

If the drum brakes are adjusted okay I would question if the length of the push rod that sticks out of the power brake booster and goes into the master cylinder is correct. Other members here should have that info for your year Mach 1.

Thats good info Mike. I had not considered that. The E-Brake does work fine but does need pushed way down to hold.

On the subject of the push rod I noticed something today I had not earlier. The push rod slipped out of alignment in the booster when I was removing the MC - as in it had slipped out of its channel inside the booster. It went right back in but I wasn't sure this was normal. It did not come out of the booster completely.
Have you replaced the check valve on your booster?
See if your brake pedal feels any different when the emergency brake is applied. Obviously, don't drive it. The rear brakes are a little suspicious at this time.

Yeah, I've had that push rod slide out of the brake booster as well. As far as I know that is normal. The length of it is critical. Slightly too short and the brake pedal will travel too far. Slightly too long and the brakes will drag or lock up. If I remember there is only about a 0.015" tolerance between too short and too long. Unless somebody here has the info for your car its best to refer to the factory service manual for the push rod length. Don't mess with that push rod length until you know everything else is correct.


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