I recently installed a nos motorcraft rabs module on my 92 f150 4wd reg cab short bed.
I have replaced the hard lines with stainless steel and the rubber lines replaced with stainless braid rubber lines, new calipers and pads, rotors,new drums,wheel cylinders,shoes. All drum hardware a few yrs old, the booster and master cylinder are three yrs old.

My problem is I have a soft/low brake petal, and got to wondering if the abs box behind the glove box needs to be plugged in for the rabs module to work properly??
I took it out three yrs ago because the abs light would randomly come on then it eventually stayed on, so I unplugged it and left it be.

From what I read the rabs is the problem a lot of the time with spongy/soft/low brake petal, but even replacing all these parts it hasn't improved my brake pedal at all.
I also think my booster is going out because I get a swoosh sound every time I press the brake and when I come to a stop sometimes the swoosh will lead to a hiss and my brake petal goes to the floor.

Also I measured the free play my petal has with the truck off, I have at least one inch of travel before any resistance is felt, I see just a tiny bit of play at the pedal connection to the booster rod, the rest is in the booster, is that normal for these trucks?

 

If the ABS module behind the glove box is disconnected then the RABS system is disabled. That in itself shouldn't cause poorly functioning brakes though. My guess is that there is still air in the system, the master cylinder is bad, or there is a small leak somewhere. From what you just said though, I would guess that the booster is having problems or the hose or check valve is bad.

 

I would replace the brake booster and the master cylinder. RABS is disabled on my truck and my brake pedal is fine.

 

I plan on plugging in the abs box today just to see if the new rabs unit gets rid of the abs light.
I wouldn't think there is air in the lines, when I changed the lines and wheel cylinders I back bleed the system first, then did the two man way of holding the break and cracking open each bleeder.
I did the rr,lr,rabs,rf,lf did that twice actually and each time I got a solid stream of fluid, I also adjusted the shoes till I couldn't spin the drums then backed them off a few clicks.
I plan on checking out the booster and also will check the check valve, I called oreillys last night to order a new booster, mine is still under warranty so it wont cost me anything but time and travel to get and install it.

 

Im going to measure the booster rod to make sure I have it set correctly, right now It seems to me that I have way too much free travel in the booster rod before any resistance is felt in the pedal.
If that does not solve the issue I will check out the master cylinder, which I would think is good because with the truck off and the vacuum depleted the pedal is rock solid.

 

I would double Check all of your connections. Even the smallest of leaks will be hard when the truck is off but slowly hit the floor when its running. You can also get a small pc of metal line with the right fittings from orlies to temporarily bypass the rabs just to make sure that is not the culprit. You will have to bleed the rear breaks again but its worth making sure thats not the issue. A bad booster typically just makes breaking harder from my experience. Maybe the master is bad again but thats doubtfull. My guess is a super small leak somwhere still or the rabs is bad.

 

The master cylinder can leak internally to where you cannot bleed it enough to get a solid pedal. However, since you have now added the info that the pedal feels fine with the engine off and vacuum depleted, I favor replacing the booster and going from there.

 

Well good news, I went to the parts store after I did a few checks.
The check valve was fine, I need a bushing on the pedal rod that hooks to the booster rod, so that adds a little play in the travel.
I also plugged in the abs box behind the glove box, and no abs light atm.
Then I installed the new booster, I bet a spent two + hrs trying to fab up a bushing for the pedal, that was a fail so I called ford and ordered one.
So to make a short story long I started it up let it idle down and I'll be damn, I got pedal.
It wasn't 100% but way better than it was, so I took if for a drive, and did several quick stops at 30-50 mph and it did great till the end, then I started to get brake fade, the front pads where smoking haha.
But I haven't tested the brakes like this since I put in the straight d44 axle, so its probably a little oil that I didn't clean good enough off of the rotors, calibers and lines were still cold.

So after I checked for leaks, and I had several, one at the pass caliper I had to tape the threads on the bleeder cause it would not stop no matter how much I tightened it, the rabs bleeder was being a pain to stop, and one line to the rabs was seeping.
One on the front of the mc I had to tape the threads cause I couldn't get it to stop seeping as well. Then the biggy, the rear line from the mc has a proportion valve on it and its seeping really good, hell I'd say more like leaking.

So when I called ford, I had him order a new one for me.
Which brings me to ask, whats inside these things and can I clean it out maybe to get it to seal up?
Pics below of the install and the valve that's


leaking.

 

that rear proportioning valve is a bear to get to stop leaking. I had to replace the one on my truck as it failed and leaked like a sieve. I taped the connection when I installed the new one and so far so good. There really isn't much inside of one just a rubber piece and a spring.
It reduces the amount of fluid being moved by using a small orifice. Mine cost about 50 bucks and there really is nothing inside to repair. Just use the new one and chunk the old one,it's not worth the headache, I tried repairing mine and it was a miserable fail. Be sure to use tape and it will be good.

I too rebuilt the whole brake system on my 93 and it works great. The only changes I made was installing speed bleeders. I would recommend putting a coat of paint on your brake booster as they tend to start rusting. I coated mine with spray on bed liner as that was all I had available at the moment and had to get the truck back on the road, so far it has worked well.

 

Thanks for the info on the valve, I was hoping ford would get it in today but no call and they close early today so that means it should be here Monday which sucks because I'm heading to txs tomorrow and not sure when I'll be home again, maybe may or june :/
Thanks for the tip about taping the threads I will certainly do that when I install it.

I actually been debating putting in those speed bleeders, I may since I know now the size required for the wheels.
As far as painting the booster, I planned on it, but its to cold out atm so when I get home next time I will clean it up and shoot some paint on it, I'm not big on the current color, and besides mine already had some rust just in the few short yrs it was on the truck.