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

[ntsqd]

Been having some problems with the H-B install in Snowball, my '96 FSB. (See my sig for a link to the build thread.) The problem is that the rear brakes slowly gain air in the system. I've bled them 3 times so far and replaced the (new) m/c once. No signs of a leak anywhere in the system except under the booster & m/c, which appears to be PS fluid, not brake fluid (feels like PSF and doesn't smell like BF). Note that this truck is plumbed by Ford to have the rear outlet drive the rear brakes.

After bleeding it has a high, firm pedal with a short travel. It is easy to modulate and the truck STOPS! Over the course of a week or more of driving it the pedal travel slowly increases and the braking effectiveness decreases. If I bleed the rear brakes I'll get a significant amount of air out of them and we're back to a firm high pedal that works extremely well. Only to start the process over.

Assuming that it isn't leaking somewhere in a way that only draws in air and does not leak fluid out (seems like a safe assumption) I'm thinking that the air is getting in at the m/c that isn't the m/c's fault (two bad m/c's in a row, from entirely different sources?). When I installed the H-B unit I noticed that the front area where the push-rod for the m/c lives was not vented. If the m/c should happen to fit tightly enough that it would see vacuum and pressure depending on where in the stroke the push-rod was and how long it had been there.

It also is slow to return the pedal after pushing on it. According to the tech guy at the vendor a slow pedal return is too much restriction in the fluid return plumbing. He told me to drill out the spittor fitting and to make the return hose longer (it's only about 7" long) so that it acts more like an accumulator. I'm not opposed to those changes, but the air getting in is my first concern. He was confident that the seemingly unvented area does indeed have a vent. I took the second m/c back off and the usual venting features are not present.

Pics of what I'm talking about, this is the H-B on Snowball:

This is the JY sourced H-B intended for the Wagon:

See that large notch in the JY part vs. the total lack of anything in the nearly new H-B unit?

Thoughts, ideas, or suggestions on how else the air might be getting into the rear brake circuit? My theory seems pretty out there, even to me.

 

[Oldtimer]

Rear disc, or drum?

 

[ntsqd]

Expedition rear discs.

 

[Oldtimer]

Issue I experienced on wife's 70 Vette.

Check rotors for runout/endplay.
Vette had .004-.006 TIR, which was enough to push pistons in & out, pumping air into brake cylinder.
Took me months to figure out, 1/2 a day to fix.

 

[dmoses42]

I thought the notch was to allow power steering fluid to drain into the engine compartment in the event of a leak in the hydroboost. But I've been wrong plenty of times...

 

[Wild horse 75]

Slow to return pedal is always the return. Look at adding an additional fitting to the reservoir so the booster and box have their own dedicated port. Tee’s do a horrible job. I’ve had brake pedals get kicked back up when you turn the steering wheel. As for the air there’s no correlation between the booster and master that could cause air to enter the system. You’ve either got a leak or as mentioned severe run out causing the pistons to bounce. It’s possible a brake hose could be allowing air into the system without showing as a leak. I would go over all you pr lines and hoses carefully and look for anywhere there’s moisture or dirt accumulated.

 

[ntsqd]

H-B has it's own return port to the remote reservoir. This is the first reservoir: https://photos.smugmug.com/photos/i...HTkmk7x5CwVXkxR7jBg2H8bKD/XL/i-QD57SkG-XL.jpg It didn't work so it got replaced with one from Lee Power Steering that also has two return ports. I'm not finding a pic of it in place.
[EDIT]: The Tech guy's wanting a longer return hose is because it rarely is full of PSF, so it acts a bit like the return is just open to the air and causes no flow restriction until it fills with fluid. The longer the hose, the more fluid it takes to fill it, which aids in pedal return.

That the notch would also show a leaking H-B unit is a reasonable idea. Wouldn't surprise me if it is for both purposes.

I'll be checking the rotor run-out, that had not occurred to me. Oldtimer, thanks for that!

My first theory is that without a vent in the front volume of the H-B that when the pedal is pushed the m/c piston moves forward, which increases the volume of the cavity. That would produce a mild vacuum, which will pull air in past anywhere it can go. With no vent that doesn't happen easily. Then when the pedal is released the m/c piston comes back reducing the volume and slightly pressurizing this volume. With no vent the pressure will go anywhere it can. The seals on the m/c piston are designed to seal pressure in the opposite direction. My idea is that this slightly pressurized air is slowly forced past the m/c piston seals.
Up until the run-out possibility it was the only idea that I could come up with that explained what was going on. It sounded pretty unlikely to me, too!

 

[ssray]

Just a thought that could aid in troubleshooting that you might confirm. Air in the rear pistons would be visible immediately when bleeding. Air in the lines close to the master would take some time to reach a rear caliper? Does that make sense or seem like your case?

 

[Shimmy]

^was thinking this too

 

[ntsqd]

The air isn't immediate. I get some fluid, not a lot, before some before I get air.

Plumbing. I reused the existing chassis plumbing on the vehicle when I removed the ABS lump. The lines were just long enough to reach the m/c area. I added this adjustable p-valve and had to make new the plumbing between it and the m/c. I also had to change the tube nuts on the existing lines to work with the p-valve. I used a Mastercool tool to make these flares. The rear flex hose is -3AN SS made from hose ends and bulk hose. Over the years I've made a lot of these hoses. No sign of wetness at either end of the hose. Plumbing on the rear axle is Cupro-Ni tubing covered in SS spiral protection with -3AN tube nuts & sleeves, and fittings. Flex hoses from the axle to the calipers are made the same way. Again, no sign of wetness anywhere.

Picture while I was still trial fitting everything:

 

[ntsqd]

Family got in the way of getting much done this past weekend. Hoping to be able to check run-out during the week.

 

[ntsqd]

Finally got to check for run-out. Put it on jack-stands, pulled off the wheels & tires, locked the OX and let it idle in 1st while eye-balling each rotor. RS rotor looked fine. LS rotor I couldn't tell. It could be the paint or it could have some run-out. Measured .025" run-out on the inner braking surface and got the same again on the outer braking surface. Then I measured the mounting face of the rotor and got about .002" of run-out. Options are replace the rotor (not trivial) or see if I can find a place to turn it. .02" seems like that's asking a lot and might go past the min thickness spec. Either way I have to get the rotor & wheel hub assembly off and that's most of the work.
@Oldtimer, thanks again for suggesting this!

 

[Oldtimer]

Not familiar with you rotor/hub assembly. Is there a chance of burr or rust between rotor and hub mounting surfaces?

 

[ntsqd]

It is a RDB conversion that I created. Expedition rear discs, a bigger version of the Explorer rear discs, on a mid 70's F-250's Dana 60. I was pretty careful about things like burrs and what-not each time I test-fitted parts. Looks like the original machining of the particular rotor let me down. It's mounting face has little run-out, but its pad wear surfaces have a lot of run-out. Just crappy parts, an all too common problem any more.

Debated having it turned and decided against that. They would need to remove enough for it to matter. I originally modified 3 new rotors for service. I just measured the spare while chucked up in the lathe, it has about .0015" run-out. I'm going to fit it instead. I've now got it mostly back together, enough that I could spin the rotors with the engine and it looks good. Next is to bleed the brakes and take it for a test drive.

A pic from the mock-up phase (those Toyota V6 calipers on the slab aren't part of anything):

 

[ksagis]

Feels to me its likely you have a very small leak someplace that is letting air in and are maybe overlooking a tiny fluid leak. Have you tried wiping all fittings really well and then put witness paper around every fitting? Then stomped on the brakes for a while and checked the witness paper?

For witness paper, I just use scott towel wrapped around the fitting with some painters tape. The tiniest amount of fluid is pretty obvious on paper.

One more, any residual pressure valves left in system? Seems unlikely to cause an issue like that tho.

 

[ntsqd]

Found that one rotor had .025"+ of run-out. Seems like the floating caliper should have compensated for that, but I've replaced it, and bled the brakes, again. Will be commuting with it this week to see what it does.

I've had it in the dirt for many miles. I would think that if it were leaking that the dust would 'witness' where it is leaking, but all of the connections are clean.

No RPV in the system, I removed that first thing. This was a first year ABS truck. All of the hyd. part of that has been removed too.

I've also drilled a vent hole in the bottom into that front volume of the H-B casting. No change in the slow return, but I didn't really expect that it would affect that.

The currently over-full reservoir isn't helping the slow return problem. My garage dedicated turkey baster is MIA, need to get another one to lower the fluid level.

 

[ksagis]

Found that one rotor had .025"+ of run-out. Seems like the floating caliper should have compensated for that, but I've replaced it, and bled the brakes, again. Will be commuting with it this week to see what it does.

I've had it in the dirt for many miles. I would think that if it were leaking that the dust would 'witness' where it is leaking, but all of the connections are clean.

No RPV in the system, I removed that first thing. This was a first year ABS truck. All of the hyd. part of that has been removed too.

I've also drilled a vent hole in the bottom into that front volume of the H-B casting. No change in the slow return, but I didn't really expect that it would affect that.

The currently over-full reservoir isn't helping the slow return problem. My garage dedicated turkey baster is MIA, need to get another one to lower the fluid level.

Any updates after turning rotor and driving for a bit? Me and @Oldtimer are curious.

 

[ntsqd]

Replacing the rotor helped, but it didn't cure it. I've not had much time to do much more, but I did put a wrench on every new connection and I found a couple that weren't as tight as I'd have liked. And that's where I'm at. My brake bleeding helper has been unavailable due to grandkid visitations. Hope to be able to bleed them next weekend and drive it to work the following week.

I did buy more hose and all of the fittings so that I can make new flex hoses. I even made a tool for inserting the brass berrys under the SS braid as everywhere I looked it was a $30 tool with $30 in shipping. Hopefully the tool will do this w/o puncturing my fingers, again. I do need to make up the spare brake hose construction kit anyway. Every vehicle that I make braided SS hoses for gets a kit with one each of each unique hose end (unless a particular hose uses two of whatever those are) and a length of hose long enough to replace the longest hose on the vehicle. That all gets "Seal-A-Mealed" in a plastic bag and stays with the vehicle.

 

[Broncobowsher]

Are the rear brakes plumbed into the front or rear port of the master?

My thoughts are the master cylinder is leaking air into the system on the return stroke. It is about the only place air can get in. Piston return spring creating a small vacuum as it pushes the pistons back. Tiny amount of air bleeding around the cup on the return stroke. My money is a bad master cylinder.

 

[ntsqd]

Plumbed as Ford plumbed it stock, rear is rear, front is front. It's not necessarily how I would have plumbed it, but I figured that I'd start out with how Ford did it and change it if I felt the need to. At this point it first needs to work, then that decision can be made. Changing it now might actually mask the problem.

I thought that too, now I'm not so sure that the air is entering the system at the m/c. It is on now it's third new m/c, two are different brands, all were sourced from different vendors. Has not made any difference. During this next bleeding I plan to look carefully at just how much fluid is expelled before the first air bubble. That might give a better idea of where in the system the leak point is.