Hey ya'll...

So probably nobody has been following, but way back about 6 mos. ago I started having the dreaded spongy brake problems. It has turned into a nearly, life-altering experience in that I have been forced to learn nearly every inch of the '92 F350 braking system in attempts to fix it. (in vain)

To catch everyone up, I have done the following...
- Replaced Front Calipers and Rubber Lines
- Completely Overhauled Rear Drums (Pads, Springs, Cylinders, etc..)
- Bent all new hard lines, replacing all connectors
- Replaced Rear Rubber Line
- New RABS
- New Master Cylinder (Upgrade to 1 1/4" bore)
- New Zero Loss Booster
- Bled the system every way imaginable with close to a gal of fluid at this point

My brakes are still ***** (spongy and pedal slowly sinks to floor) and I'm losing my sanity...

Someone mentioned to me about testing for vacuum and I may have a new lead. I hooked up a vac gauge to the distribution block on the drivers side near the Master Cylinder and I only got around 15" or so. (This bleeds off to zero less than a minute after engine shutoff) Everything I've been reading says this is garbage and needs to be higher. So I'm wondering where to go at this point? There are no warning lights on which I understand there should be if its this low. (its possible I have *multiple* issues going on as the PO of this truck was a hack)

Can anyone provide step-by-step how to isolate *specifically* what is the issue with vacuum? I'm looking to isolate booster, vac pump and lines maybe? (one by one)

As always, thank you in advance...

 

The whole point of the vacuum booster is to make the brake pedal easier to apply, so if you're brake pedal is spongy and sinking to the floor, a loss of vacuum to the booster is not the cause. 15 in*hg of vacuum at idle may not be the best, but for an aging engine, it's more than sufficient.

If you pump the pedal with the engine off, does it build pressure? If so, will it hold pressure or does it still sink to the floor?

A spongy/low brake pedal is only caused by a few things -

1) Air in the lines.
2) Drum brakes out of adjustment.
3) Calipers getting "pushed back" by warped rotors.
4) Brake pedal linkage out of adjustment (but it would have to be really out of adjustment - as long as you don't have more than a couple inches of brake pedal free play, the linkage is fine).

You're not gonna like me for asking these questions, but it has to be done because even the simple things can be overlooked -

1) Did you bench bleed the master cylinder before you installed it? Even a shallow mounting angle can be enough to trap air at the end of the bore if you don't bench bleed it before installation.

2) Are you using pressure bleeding or manual bleeding? If manual, sometimes it helps to allow the brakes to gravity bleed for a few minutes beforehand.

3) I didn't see any mention of new drums. Sometimes the drums can be so worn out that even new shoes and hardware will not allow adequate contact to create "push back" on the wheel cylinder and build hydraulic pressure.

4) Are you sure you installed the rear brake shoes and hardware correctly (didn't accidentally swap left and right side adjusters)?

Don't rule out any possibilities.

 

Also, one way to isolate a general area for the cause -

Using only a pair of hose pinching pliers (see below links for options - vice grips are a no-no) -

1) Pinch off the rubber hose to the rear brakes. Apply the brake pedal - is it firm? Yes - problem is isolated to the rear brakes. No, proceed to step 2.

2) Pinch off the rubber hose leading to the left or right front brake. Is the pedal firm now? Yes - problem is isolated to that brake. No, proceed to step 3.

3) Pinch off the rubber hose for the opposite brake. Is the pedal firm now? Yes - problem is isolated to that brake.

If the answer to step 3 is no, there's probably air still trapped in the lines or the master cylinder.

Types of hose pinch off pliers (not recommending the brands, just the types) -

https://www.sears.com/schwaben-locki...Fc_ZwAod3gQNNQ

https://www.homedepot.com/p/GearWren...FVLZwAod8hQGxA

https://www.carid.com/images/oem-tools/items/24432.jpg

 

DBGrif - Thank you for taking the time to reply with all of that, much appreciated!

The brake pedal is very solid and doesn't hardly move with the engine off. When I stand on it and start the engine, there is about a 1-2 second pause and then the pedal slowly starts sinking to the floor. (maybe 10-15 seconds) From that point, it feels spongy and doesn't seem to "recover" much at all. I replaced my booster with the "zero loss" unit and it was my understanding there should be no drop at all.

To answer your questions...

1) Bench bleed - Yes. I did a full bench bleed before I put it on. (being careful not to push the piston too far in and damage the internals)

2) Pressure bleeding. I am not getting any air out of the system when I do it, which leads me to believe its bled completely.

3) Drums are the one thing I didn't replace yet actually. (good catch) My understanding is that they would have a "groove" if they were worn excessively. Mine looked to be in pretty good shape and came off fairly easily so I figured they were still good.

I'm going to try the hose pinch tonight and double check the adjusters to see if I can narrow things down.

 

For clarification purposes - with the engine off, do you have to pump the pedal to build pressure? If so, it could indicate that the brake shoes are having to travel farther than they should be to contact the drums.

I'm not too familiar with zero loss boosters, but with the way you're describing the pedal being firm with the engine off, and then dropping after you start the engine, it sounds like normal booster operation to me. That's actually a standard practice for testing them.

Does the RABS module have a bleeder valve on it, and if so, when are you bleeding it? The bleed sequence I found was RR, LR, ABS valve (if equipped), RF, LF. In theory, since you're using pressure bleeding, it shouldn't matter, but if the RABS module does have a bleeder screw on it, bleeding out of sequence could be causing the problem.

 

I have a quick question about bleeding the RABS valve. Do you just bleed it at the fitting or do you do the outlet, and inlet lines separately like you bleed the forward and rear lines at the top after replacing a master? The order you gave is straight out of any repair manual but I could not find how to bleed the RABS valve in my Chilton manual. Also, NCFarmLife, when you replaced calipers on the front, did you mix them up? If they are mixed up the bleeder maybe upside down in relation to the brake like. You'll see clear fluid all day long but never any air. I think Eric the Car Guy has a video on this mistake when he installed disc brakes on the rear of his car.

 

My standard sequence is passenger rear, driver rear, RABS, passenger front, driver front... But I've tried just about every combination imaginable at this point. (also bled the RABS with and without the spring installed) I have not tried bleeding the fittings on the RABS, just using the bleeder screw. (I didn't want to introduce any air in at the fittings)

Its pretty firm with the engine off. I might be able to pump once, but it doesn't really want to go anywhere. I'm going to try adjusting the cylinders again farther. This all seemed to start when I did the drums. As it is now, I can spin the tires with the truck up in the air and they stop within about a turn, but maybe this is still too loose?

If you mean the bleeders pointing down, no. I already thought to check that.

Appreciate you guys sticking through this with me. If I can't get it this week I'm finally admitting defeat and taking it into the shop. I'm over working on it at this point....

 

Just use the bleeder screw. The only reason I can see to use the fittings to bleed the valve is if the bleeder screw broke off or won't come loose.

There's other drag on the wheel on account of the differential and the tire on the opposite side, so unless you're going to unhook the driveshaft and remove the opposite wheel, spinning the tire isn't going to be terribly accurate.

The way that I always adjust drums is to tighten them up until you can't turn the wheel. Then I back the adjuster off 1 turn - it can be hard to know when you've actually backed it off one turn, so I use a pick, pencil, marker, etc - whatever will fit into the hole in the backing plate and leave an identifiable mark on the adjuster wheel. I've even swiped a bottle of my wife's fingernail polish before - and then promptly replaced it with one that was not used for automotive purposes.

I also did some more digging on bleeding the brakes on this truck and found something interesting regarding the sequence. I apologize for missing this before - I just did a quick scan of the document instead of taking the time to read it thoroughly.

The information that I found states the following:

Before bleeding system, exhaust all vacuum from power unit by pumping brake pedal several times. Bleed master cylinder first by loosening fittings on the primary and secondary lines one at a time. Then bleed the RABS electo-hydraulic valve (module). Then bleed at each wheel, RR, LR, RF, LF.

If you're having a hard time pumping the pedal with the engine off, pull the check valve out of the booster - that will deplete the vacuum in the booster.

 

Fair enough... I'm going to try your method next.

This is very interesting!!! This is the first time I've heard of this sequence. Everyone has told me the RABS gets bled *AFTER* the rears. Can you tell us where you found this?

If this works, I owe you big time...

 

I'm still digging through service information to find out more about why, but I found this sequence suggested after replacement of the RABS module.

I have access to identifix.com at work and prodemand.com from when I was in school, and usually with these older trucks (or any vehicle with hard-to-find service info), I'm jumping back and forth between them to check and double check service info.

I think the sequence I found on prodemand.com was listed to be performed after replacement of the RABS module, but I didn't get a chance to verify it on identifix at work today.

What caught my attention was making sure the vacuum was depleted in the reservoir - I can see how having vacuum present in the booster would keep the piston in the master cylinder "pulled forward" during bleeding (even though it's not supposed to work that way).