Brake system air induction; was: Odd HydroBoost (?) Problem

[ntsqd]

More bleeding after finding some of the connections looser than I'd like. It was a small volume of fluid, before the first air bubble. Maybe or maybe not all of the caliper's fluid volume, but close.

I noticed something that I've checked for in the past and found nothing. This time I did. This truck had ABS, first year. It is gone now, at least all of the parts in the brake plumbing. Because of it's ties into the ECU the brain is still there. For whatever reason the rear drum brake's residual pressure valve (RPV) is located in it's own housing that screws into the master's rear output. Each new master has come with an o-ring to replace the existing o-ring when you transfer this assembly to the new master. I had to take apart the RPV to defeat it while retaining the RPV's body because the output port itself has no way to seal a tube in it*. Today I noticed a small spot of liquid on the ground under the master. Looking and feeling around all of the connections I found, for the first time, some wetness at the junction between the main body of the RPV and it's "cap". It tightened a little more, we'll see if that was enough.

*Seeing that wetness got me to thinking about creating just such a one piece part. One that will screw into the master and have an inverted flare tube fitting without it having the whole RPV body.

 

[ksagis]

This was an area that you hadn't noticed wetness before? I've had best luck wrapping fittings with a little bit of scott towel and leaving it for periods of time to find tiny leaks that usual "wetness" checks might be elusive. The tiniest bit of brake fluid shows on a scott towel, but can be hard to feel.

Does the volume of fluid correspond to the where you found the loose fitting?

Getting RPV can only be goodness.

 

[ntsqd]

I had looked and felt that area every time I had cause to look at all. Since I had opened up the RPV to defeat it's function it was suspect until proven not to be. Until this morning, no observed wetness of any kind.

There were several of the -3AN fittings that went another ~1/16th turn tighter. No sign of fluid at any of them except for maybe one of them. There is one that I may try the Scott towel trick on, just to be sure that it isn't leaking. There's an odd stain on the POR-15 near it that only makes sense if it were leaking badly, but I've never seen it wet either.

Bled the brakes, again!, and took it on a ~200 mile drive over a mountain pass and back. Brakes feel as good as right after bleeding them. Will be driving it in next week's commute to see if this lasts.

 

[nvrstuk]

Hope you're good TS!

 

[ntsqd]

Thanks Brian!

On the drive in this morning I noticed that I wasn't seeing the slow return of the pedal. Perhaps that only happens when there is air in the rear brake system? As might be imagined, when there is air in the system the pedal travel is considerably longer.

 

[nvrstuk]

I would think from a physics standpoint - that air in the rear lines would help the pedal return quicker.

Since fluids do not compress and air does, the air is under pressure in the line. When you let off on the brake pedal, that "residual" air pressure in the rear line would want to help return the brake pedal quicker.

 

[ntsqd]

I suspect that it acts a bit like a negative spring in a suspension fork. But if it only happens when there's air in the rear system then fixing one problem fixes both. I hope.

 

[nvrstuk]

Hope so!

 

[ntsqd]

Brake pedal travel has increased enough that it's clear that it now has air in it again. 277 miles per pint of brake fluid isn't very good.....

I found the Ford p/n for the OEM RPV and found a vendor supposedly with inventory. These do not come with a new or rebuilt master cyl. They screw into the side of the m/c with an o-ring seal. New m/c's supply a new o-ring only. Once the replacement RPV assembly is here I will be taking the the existing one back apart and looking into how it can be made to be one piece (they are a body, a cap, and some pieces inside). This will likely either involve silver-solder or silicon-bronze filler rod.
An aspect of the RPV body is that it has some considerable interior volume. Ever since gutting it I've wondered if it could be trapping air as bleeding the rear system isn't fast or easy. (I'm 1/2way thru the third quart of brake fluid in dealing with this!) The next iteration of that part will be a good bit smaller and not have that volume.

 

[nvrstuk]

Need to ask why the need for a RPV?

Since that could be the culprit, why don't you just bypass that and get a in line one from summit? I have one I could send you if I could find it. It is two p s i.

I bought it when we were all converting to discs 30 yrs back and thought I might need it. And I never did

 

[ntsqd]

The system doesn't need or want an RPV, but the m/c is made in a way where the body of the OEM RPV needs to be there for it to the tube to connect. It is a whole separate thing that screws into the side of the master and not just a check valve under the inverted flare conic seat. No idea why Ford did it this way, the only guess that I have is that it is somehow related to this being a first year 4WABS. I took the existing RPV apart and removed all of the guts so that it is now basically an adapter between a funky o-ring boss port and an inverted flare tube.

This Cad plated thing stuck to the side of the m/c body:

Unlike the unit in the pic, mine is two pieces, the main body and a 'cap' that screws into the top of the body. Where I was finding fluid leaking from was between the body and the 'cap'. As best as I could tell the body to m/c is not leaking. My plan is to see if I can shorten the RPV assembly to reduce or eliminate any volume that might now trap air and to braze or silver-solder it so that it can't leak where the cap is screwed into the RPV body.

 

[nvrstuk]

I like a good plan!

 

[Wild horse 75]

Could you have an adaptor made to eliminate the RPV adaptor?

 

[nvrstuk]

Send us a good pic or 2.

 

[ntsqd]

An adapter is basically what I've made out of the current RPV, I'm just thinking that I can make it shorter, with less internal fluid volume, and one piece - no screw-in cap by brazing or silver-soldering the two pieces together. I ordered the second RPV so that I have a back-up if I totally flub modifying this one.

 

[ksagis]

Something still doesn’t compute to me. If air gets in, then either fluid leaks out and MC height stays the same, or MC fluid height goes up.

An air bubble at the RPV at MC would have to be drawn all the way through system before coming out at bleeder when you bleed it. Do you get quite a bit of “clear” fluid and then bubbles? Given you found wetness there that could be it.

One more thought, wonder if a bubble that close to RPV could get sucked back into MC after bleeding and during driving. (Seem to recall you swapped MCs a few times?)

 

[Oldtimer]

What size is ORB end of RPV?
Convert to -3AN at MC?

Adapter, -3 ORB Male»-3 AN Male, AL BK (ADF-R3J3): AN JIC Male » AN ORB Male Adapters

Purchase Adapter, -3 ORB Male»-3 AN Male, AL BK or shop other AN JIC Male » AN ORB Male Adapters and Racetronix products.

www.racetronix.biz

 

[ntsqd]

This is the second new m/c. The other day was the first time that I had found fluid at the RPV. Prior to that it had been dry when I checked it. Tellingly, I was able to check it immediately after driving it this last time. Prior checks were hours later. The amount of fluid present is/was minuscule, it is ever so barely leaking. Level in the reservoir has been going down slowly. I've known that there was a leak, I just couldn't find it.

Good thought, but it is not a normal ORB port, it just uses an o-ring to seal the base of the RPV body into the m/c.

Hard to gauge just how much fluid volume is coming out. How do you quantify the fluid volume of the plumbing between the m/c and the brake caliper? Can estimate it, but how to compare that to what comes out? Roughly the third pedal pump of my ever faithful brake bleeding assistant is when I get the first bubbles. I am using the "peanut butter bleeder" to catch the fluid:

 

[ksagis]

This is the second new m/c. The other day was the first time that I had found fluid at the RPV. Prior to that it had been dry when I checked it. Tellingly, I was able to check it immediately after driving it this last time. Prior checks were hours later. The amount of fluid present is/was minuscule, it is ever so barely leaking. Level in the reservoir has been going down slowly. I've known that there was a leak, I just couldn't find it.

Good thought, but it is not a normal ORB port, it just uses an o-ring to seal the base of the RPV body into the m/c.

Hard to gauge just how much fluid volume is coming out. How do you quantify the fluid volume of the plumbing between the m/c and the brake caliper? Can estimate it, but how to compare that to what comes out? Roughly the third pedal pump of my ever faithful brake bleeding assistant is when I get the first bubbles. I am using the "peanut butter bleeder" to catch the fluid:

Well, first principles seems to check out.

Assuming 0.25” brake line to rear brakes, 0.040” wall line, and 92” of line to back, 36” to caliper, and 1.25 factor for not straight run gives about 4 cu in of brake line volume.

1.375” MC is 1.48” area and maybe 0.5” of MC stroke depending on pedal ratio is 0.74 cu in per stroke.

Super rough math comes up with 5.33 strokes to move bubble to caliper which doesn’t compare too badly to what you say above and my fairly gross assumptions.

I’m willing to bet you’re on right track. Though that is high spot and air introduced someplace else might migrate up there. The wetness you found is pretty good indicator tho.

Bet you’re getting close to root cause!

 

[nvrstuk]

I'll bet the rear brake line is half that dia but I like all your math so he's down to less than half those pedal pumps