1990 F150 Brake Woes

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Little history about the truck here in my thread here: https://bobistheoilguy.com/forums/threads/sludgy-1990-f150-5-0.332771/

My son has been working on this truck for almost a year and has put very few miles on it as the problems keep coming.
Lately we are fighting with the brakes. He has summed them up on another forum and hope someone could have a read and offer some advice.
https://www.ford-trucks.com/forums/1669225-1990-f-150-persistant-brake-issues.html

Every time we bleed the brakes there are bubbles at all 4 corners. I am suspecting the RABS and just want to bypass it although not sure how that can affect the fronts. What is left...:oops:

Son has worked so hard and it just sits in the garage for the most part.
 
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the bleeders, I like to pull them & wrap a little teflon tape on, wrapping CCW as looking at the screw top, so the tape is drawn into the threads in the wheel cylinders/calipers. Helps keep air from sneaking past those threads. Just the threads near the end, you don't want teflon drifting around in the interior chambers.
 
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the bleeders, I like to pull them & wrap a little teflon tape on, wrapping CCW as looking at the screw top, so the tape is drawn into the threads in the wheel cylinders/calipers. Helps keep air from sneaking past those threads. Just the threads near the end, you don't want teflon drifting around in the interior chambers.
They have a seat like a valve in a head. Wrapping the threads does not do a thing for sealing.
 
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My guess he is not bleeding correctly. I would bleed starting with RR, LR, FR, FL. From his post, looked like he was starting with LR. Is he holding the pedal down as someone tightens the bleeder screw? Maybe if you provided more detail of how he is bleeding his brakes.I have RABS with my 87 F150 and have bled the entire system by myself using a precut piece of wood, wedged between the seat and brake pedal to hold it down as I tightened down the bleeder screws and it was easy and took around 15 minutes to do all four wheels.
 
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They have a seat like a valve in a head. Wrapping the threads does not do a thing for sealing.
when they are cracked 'open' for bleeding, air can get drawn through the threaded portion. The brake fluid moves through the hose, but air gets drawn in through anyplace along the way. It crops up with gravity bleeding, the hose is immersed in fluid but air can get pulled through the threads with the bleeders cracked open unless you use teflon dope or tape to block it.
Not talking about sealing the bleeders when tight & drawn down, only when they're partially open to permit a bleeding operation.
 
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when they are cracked 'open' for bleeding, air can get drawn through the threaded portion. The brake fluid moves through the hose, but air gets drawn in through anyplace along the way. It crops up with gravity bleeding, the hose is immersed in fluid but air can get pulled through the threads with the bleeders cracked open unless you use teflon dope or tape to block it.
Not talking about sealing the bleeders when tight & drawn down, only when they're partially open to permit a bleeding operation.
Air could only be drawn in when the pedal is released while the bleeder screw is loose.. I never had to use teflon tape on any bleeder screw.
 
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I know the MC was replaced, but I wonder if the replacement is good. I only scanned the thread but if the pedel is slowly sinking then I am not sure that the bubbles seen is a bleeding problem.
 
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The first thing you have to figure out is where the bubbles are coming from.

I would be subject to start over on the bleeding rr to lf is the usual recommendation, but some like to bo it lf to rr. Heck do it both ways

Talk to us about the RABS, what did he take apart?

RE the other forum - I quit reading when they started going on about the pinion angle. 90 percent of what they are talking about can’t cause bubbles in the system or a sinking pedal.

IMO once the air is out sinking pedal = bad master cylinder (once no leaks verified.
 

cbrf3

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So we have been bleeding using 2 person, vacuum with a Mity-vac and gravity methods. RR, LR, RF then LF. Tried about 10 times , get bubbles every time at every corner. Master cylinder has been replaced and seems to hold pressure and feel pretty firm in the garage with the truck off. Did have to pump it a few time in the garage truck off to built the pressure then quite firm and held. Starting the truck and pedal seems decent and if we remove the vacuum hose from the booster the truck dies.

Here is his original post from the F150 forum:

This is everything I have done to my trucks brake system:
  • Replaced rear shoes and all hardware, replaced the wheel cylinders, bled the system and set rear shoes correctly .
  • Replaced all rubber lines since the old ones were probably 31 years old and bled the system again.
  • Replaced front brake calipers and bled the system again.(rear l, rear r, front r, front left)
  • Replaced the Master Cylinder (bled on bench and on truck) and replaced the brake booster and again I bled all the brakes.
  • Took the spring out of the RABS module to bleed it and then put the spring back in after bleeding. However I noticed while the cap was off the module, brake fluid was leaking out from where the spring sits.
  • I replaced parts because they were worn and or original parts.
I am out of things to either check or replace as I still have a sinking pedal. I have bled them with 2 people, gravity and vacuum but I seem to have air with every bleed. The brake pedal feels decent with the truck off but sinks once stated up and then slowly returns.

Is my RABS module bad, or is something else going on? I have gone over the brake system with a fine tooth comb at this point and haven't seen any leaks that could be introducing air. The truck is a Georgia truck and has hardly any rust on it and the rust it does have is surface only. Should I bypass the RABS module, and if so with what part(s)?


Booster rod has been adjusted for almost no play up to the MC piston, we have used a MC bleeder kit multiple kit after the MC was bench bled.
Fliud level does not go down and we keep it full when bleeding.

Here is his last post, RABS is the rear abs that the truck has:

So I saw on some other threads about bypassing the RABS module, which I did so last night since it didn't work since I got the truck. I gravity bled the brakes too. This afternoon I decided to test the brakes in my neighborhood. When I went to fire it up the brake pedal was firm, fired it up felt pretty good, not squishy like it was. Went to back the truck out and the brakes went to the floor. About soiled myself since I live on a hill I promptly pulled right back into the garage, the brakes had nothing for about 98% of the pedal travel then all at once, but not locking up.

I also ran a test on both the master cylinder and the booster and both are working fine. No leaks what-so-ever, not even a slight one. I even made sure all the brake lines were tight to verify no leaks, before I bled the brakes, fluid level didn't change either.

Does anyone else have ANY idea what is going on??


The RABS has a chamber in the front with a spring, my Son removed the spring and sealed the unit up, said to defeat the ABS action by doing so. We have read of issues with these truck with the RABS and are thinking about trying to bypass it but seeing air is always in the front corners as well makes us wander if that is where the air is being introduced.
 

cbrf3

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I also want to mention that the ABS light has been on for a year, since he got the truck. I believe the rear ABS just senses if the rear wheels stop turning and then disables the rear brakes. The F450's did not have the RABS and simply had one line in from the MC, a short block/pipe and out to the rear brake line. The reason I would guess is the F450 would have more weight on the rear.

We are going to start over by bleeding the MC then all the corners. I will hold the pedal down until my Son closes the bleeder/MC line. The RABS has a bleeder as well. Typo above but we are typically told to bleed RR, LR, RABS, RF then LR. on this guy. Last night I pushed the brake pedal, truck off, to see if the pedal would sink or if there were any bubbles/turbulence in the MC reservoir. It took 4 pushes on the pedal until it firmed up and it was engaging at the correct level.

The brake fluid is pristine at this point as all the old fluid is long gone after 10+ bleeds. Is the fluid coming out considered trash if we decide to gravity bleed at some point, cannot be put back into the Master Cylinder? Truck does not leave the garage at this point.
 
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Never put back fluid that has been bled out. Brake fluid is not expensive.

The problem with only pressing the pedal in the garage is that you can't tell when the brakes start to act. It could be that you think the pedal is going down a long way before it doesn't press any farther, but if you did that with the truck moving you'd have the wheels locked up long before that point. If you can stop at all on a test drive, do several stops moving backwards to self-adjust the rear drums.
 

cbrf3

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Never put back fluid that has been bled out. Brake fluid is not expensive.

The problem with only pressing the pedal in the garage is that you can't tell when the brakes start to act. It could be that you think the pedal is going down a long way before it doesn't press any farther, but if you did that with the truck moving you'd have the wheels locked up long before that point. If you can stop at all on a test drive, do several stops moving backwards to self-adjust the rear drums.
Too scary to take it on the road. Last trip back out of the garage had the pedal almost to the floor before the brakes engaged. Have tried to run the truck in reverse before several times and the fronts finally would lock. Rear brakes seem to be tight/adjusted, a bit of drag.
 
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First, make totally sure the calipers on the rights sides (bleeders must be at the top). Then, I would get the truck up on jack stands and inspect every bit of brake line and hoses for leaks. If nothing is suspect with the lines, did you bench bleed the new master cylinder? If yes, then I would try a new master cylinder as they are cheap.
 

cbrf3

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Are the calipers on the wrong sides?
The calipers/wheel cylinder have the bleed nipples at the top. When the front calipers were removed old nasty fluid came out even with all of the bleeding. Everything inside of the rear drums are new, new front calipers, all new rubber hoses, replaced MC and booster. Everything is tight.

Air comes out of the MC lines every time we bleed it and air is found at every corner with every bleed.
We are about to bleed again. It is Saturday after all. 😢
 

cbrf3

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First, make totally sure the calipers on the rights sides (bleeders must be at the top). Then, I would get the truck up on jack stands and inspect every bit of brake line and hoses for leaks. If nothing is suspect with the lines, did you bench bleed the new master cylinder? If yes, then I would try a new master cylinder as they are cheap.
Bench and bled on truck. Bleeders are at the top. Every inch of the brake system is dry and tight.
The MC was removed and is dry at the rear seal. Leaking internally?
 
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By now you probably have a lot of brake fluid all over. I would clean the outside of the master cylinder, brake lines, ABS units, backing plates, calipers, etc with brake cleaner and let it dry. Then have one person continuously pump the brakes while the other examines everything from soup to nuts for wetness. Or... go straight to a new master cylinder if time is worth more than parts to you. There's only so much a man can take before getting the parts cannon out.
 

cbrf3

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By now you probably have a lot of brake fluid all over. I would clean the outside of the master cylinder, brake lines, ABS units, backing plates, calipers, etc with brake cleaner and let it dry. Then have one person continuously pump the brakes while the other examines everything from soup to nuts for wetness. Or... go straight to a new master cylinder if time is worth more than parts to you. There's only so much a man can take before getting the parts cannon out.
He has been wiping it down clean but we will start with the inspection. I think we might need to buy DOT 3 by the drum at this point.
Suppose if the MC was bad out of the gate it would explain chasing our tails for about a month. It would be nice to be able to drive the truck some as well.

Thank you for all of the suggestions everyone.
 
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