A few months ago I did a "Full Monty" on the front end of Old Brickie. Leveling springs, shocks, radius arms, manual hubs etc.etc. I changed the calipers, pads, front brake lines and master cylinder (New Motorcraft). Prior to this my brakes were working fine (as far as I can tell). I vacuum bled the brakes and master cylinder. Tried the RABS valve but got nothing. I've been told that needs to be pressure bled from the master cylinder.
Since then I am experiencing strange behavior. I get similar behavior to what is described on this 7.3 diesel forum post.
Link to Thread
"I had a braking problem on my '96 F250 PSD. When you'd apply the brake all felt right in the world until just before it was suppose to stop, when suddenly the pedal would release and travel all the way to the floor. If you went ahead and romped on it the truck would stop, but made a funny jerking sound which you could feel as well as hear."

If I pooch around and brake similarly the brakes stop fine. If I'm a bit more spirited and push the pedal hard the pedal "gives way" a bit and the truck still brakes although there is too much pedal movement for sure. I'm re-visiting this now after other work. I did headers, upper intake manifold but I've checked vacuum connections, although I have not checked the vacuum to the brake booster. I need to swat up on that. The truck is running really well except for a slightly high idle.

All I can say about the brakes is I have that very periodic RABS warning but its quite infrequent. No codes.

Any pointers? Could it be the brake booster or is it related to changing the master cylinder. The pedal feel strange - not sure I'd call it a jerking sound but definitely odd.

I'm thinking I might have to bench bleed the Master Cylinder again and re-install and check the pushrod adjustment on the brake booster. I see Haynes 9-12 says " Some common symptoms .....excessive brake pedal travel accompanied by a groaning sound from the brake booster".

 

The usual cause of this is a bad brake booster.

 

I thought the pedal went hard when the brake booster was faulty?

 

It can also do that. If you do a search on here, you'll see it's a pretty common failure. In fact, it happened on my 1994 F350.

 

This morning I did an operational test and a vacuum test.
OP Test - Released pressure with engine off by several pedal depressions. Pedal hard. Turned on engine while depressing pedal. Pedal fell away correclty indicating vacuum assist.
Vac Test - Ran engine. Turned off. Waited 90 seconds. Several light pedal depressions. Pedal gradually became firm after 3 or so depressions. Seems ok. However pedal seems to make "squishy noise" when vacuum assist is working.
I have not checked the actual vacuum reading or the functionality of the check valve.
Seems an odd coincidence that this happened after I did brake work and changed master cylinder.

Update: After 30 years I 'm going to change the booster and then power bleed the system again.

 

UPDATE : I changed the brake booster which was clearly shot. The braking improved but I still had that "squishy" feel when the brake pedal was fully depressed and my intermittent RABS flashing light was coming on still. I was able to get the flash codes from grounding the test connector. 10 flashes.

After following the diagnosis charts I determined that the RABS computer module was bad. So.................I ordered a NOS one off Ebay. Installed it last week. Unbelievable...brake squish all gone and braking beautifully. On a dime. However - I now have a full time yellow RABS light on. No red brake warning light. This time it's 12 flashes.
According to the Ford RABS manual, Flashout code 12 indicates "Loss of Hydraulic Brake Fluid For One Second During Anti Lock Stop". I have no fluid loss. Searches here and on YouTube seem to indicate wiring faults.

I have followed Diagnostic Test D - "Yellow Light On, Red Brake Light Off" test sequence. I followed the test to D6, which tests "Voltage at the fluid level circuit". Testing Pin 2 on the module connector I get 12 volts so I follow the chart & jump over test D7 and go to D8. D8 test passes which says go to Step 9 which is replace the computer module and re-test.

I go back to test D7 which I supposedly did not need to do, which Is the Fluid Level Sensor Wiring test at the master cylinder. I'm getting inconsistent voltage on the center wire. I need to check the connector. But I got consistent 12 volts on pin 2 at the computer connector in test D6 which seems to be voltage coming from the master cylinder (I "think" - which is why the diagnostics said to skip over the master cylinder test D7, if that makes sense?)

2 questions:
1. Can someone describe the electrical flow/voltages at the master cylinder 3 wire connector?
2. Is there a source for replacing that connector?

As a reminder - During brake restoration work I have replaced:
1. Master Cylinder (New Motorcraft).
2. Booster (Cardone).
3. RABS Computer Module (Ford NOS).
4. Brake Lines, Pads etc.

What I have not done yet is successfully bleed the RABS Valve. I still intend to pressure bleed the system even though the brakes are working exceptionally well. I spoke to Cardone once, who rebuild the RABS valves, and they suggested pressure bleeding works but vacuum bleeding does not - as it pertains to the RABS valve.

Thoughts?





Update: I have located the connector part #. It is E8AB-14489-A and I secured 1 off Ebay.


Still looking for feedback on the 12 flash code issue.

 

/

 

YELLOW REAR ABS LIGHT ON AND RED BRAKE LIGHT ON -- TEST C

C1: LOW BRAKE FLUID
C2: MASTER CYLINDER FLOAT
C3: DIODE/RESISTOR ELEMENT
C4: DIODE/RESISTOR ELEMENT PARKING BRAKE AND/OR DIESEL LOW VACUUM SWITCH REMOVED
C5: ANTI-LOCK VALVE SWITCH
C6: MASTER CYLINDER SWITCH
C7: BRAKE LIGHT WIRING

C1 LOW BRAKE FLUID:
Check the brake fluid level at the master cylinder reservoir.

Brake fluid level OK
GO to Step «C2».

Brake fluid level low
CHECK for fluid leaks in vehicle brake system and repair as required. Fill master cylinder to required level.
================================================== ==========================
C2 MASTER CYLINDER FLOAT
Check master cylinder float for buoyancy:

-- Remove cap from master cylinder reservoir.
-- Using a clean steel implement, push down on float in reservoir.

Float moves down
GO to Step «C3».

Float does not move down (sits at the bottom of the reservoir)
REPLACE master cylinder reservoir. REFER to «Section 06-06», Brake Actuation, Hydraulic, for procedure.
================================================== ===========================
C3 DIODE/RESISTOR ELEMENT
Check for proper functioning of the diode/resistor element.

-- Turn ignition key to the on position.
-- Check parking brake and release if applied.

Both the REAR ABS and BRAKE warning lamps go off
REPLACE RABS diode/resistor element.

Both the REAR ABS and BRAKE warning lamps stay on
GO to Step «C4».
================================================== ===========================
C4 DIODE/RESISTOR ELEMENT PARKING BRAKE AND/OR DIESEL LOW VACUUM SWITCH REMOVED

Continue to check for proper functioning of the diode/resistor element.

-- Remove the parking brake switch and the diesel low vacuum switch, if so equipped.

Both the REAR ABS and BRAKE warning lamps go off
REPLACE RABS diode/resistor element.

Both the REAR ABS and BRAKE warning lamps stay on
GO to Step «C5».
================================================== ==========================
C5 ANTI-LOCK VALVE SWITCH

Obtain the flashout code as described in Diagnosis and Testing in this Section.

Flashout code is obtained
REFER to «Flashout Codes Charts» in this section.

REAR ABS and BRAKE warning lamps stay on steady
GO to Step «C6».
================================================== ========================
C6 MASTER CYLINDER SWITCH

Check for proper functioning of the master cylinder fluid level indicator switch:

-- Remove the connector from the master cylinder.
-- Connect a jumper wire between the two purple/white wires in the connector.
-- Turn the ignition key to the one position.

REAR ABS and BRAKE warning lamps stay on
GO to Step «C7».

REAR ABS and BRAKE warning lamps go off
REPLACE the master cylinder reservoir. REFER to «Section 06-06», Brake Actuation, Hydraulic, for procedure.
================================================== ==============================
C7 BRAKE LIGHT WIRING

Check for shorts in brake light wiring.

-- Disconnect module harness connector from module.
-- Turn ignition key to the on position.

REAR ABS light goes off and BRAKE light stays on
CHECK for short to ground in the 977 circuit. REFER to «wiring diagram».

Both REAR ABS and BRAKE warning lamps go off
REPLACE module.

 

THANKS for pointing out all the relevant charts!
My issue is that the Yellow ABS light is on and the Red Brake Warning light is OFF, and that threw me off.
Interesting that this morning I do not have the Yellow ABS code . More importantly the Red Brake light did not self check. I should have picked that before. It does come on with the parking brake pedal.

So before doing the jumping at the connector (C6 test) I wiggled the master cylinder connector and I found out that when I push the connector in firmly the Red brake warning light does not self check. When I pull the connector away from the cylinder (still connected but has a little play), the Red Warning light self tests. I could do it repeatedly - at least 6 times - and duplicate the condition. So I need to begin by fixing the connector.

Mea Culpa! - had I started the troubleshooting properly I would have picked up the issue. In my excitement at getting the 12 flashing codes I mistakenly jumped straight into the flashing code diagnostics. Beginners mistake.

The connector problem has likely been there all along. Mistake - I jumped to test D (Yellow light On, Red Brake Light off) instead of following the Diagnostic charts.

Its interesting that in theory you could jump the 2 purple wires on the connector (C6), see the brake warning light go off and therefore assume the master cylinder was bad. The connector integrity needs to be checked carefully - something you don't see in the charts.



I could probably dismember the connector and fix it but as I found new ones on Ebay cheap I'll wait a few days for it to get here. We'll see if all is functioning correctly then.

Connector

 

What manual are those pages from?

 

Forgot I got this some years ago and it was on the shelf . Has more info than Chiltons, Haynes and EVTSM. RABS 1 Printed in 1991. CTP1991-4 on back page.

Some on Ebay