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

[ntsqd]

3/16" OD with an .035" wall thickness is the common brake tubing set of dims. 104" wheel base, likely closer to 110" of brake line. So roughly 3.03 in^3 of brake line volume.

RA lists two different m/c bore sizes, 1-1/16" and 1-1/8"; I'm pretty sure that I was careful to order the 1-1/8"
At 1/2" of m/c piston stroke that comes out to be .497 in^3 per stroke. That makes it 6 strokes to push the fluid that started in the m/c out of the bleed screw.

What I was alluding to is that I have no way to effectively measure the volume ejected before the bubbles arrive. I use the peanut-butter bleeder to capture the ejected fluid (rather than making a mess) and to insure that no air is drawn back in during the return stroke. FWIW I load the bleed screw threads with dielectric silicone grease whenever I put a new caliper on a vehicle, no air is likely getting in on the return stroke by that means.

 

[nvrstuk]

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.

Re-reading this thread.

TP is what I use. Lighter weight and shows better if it absorbs even one drop.

 

[ksagis]

3/16" OD with an .035" wall thickness is the common brake tubing set of dims. 104" wheel base, likely closer to 110" of brake line. So roughly 3.03 in^3 of brake line volume.

RA lists two different m/c bore sizes, 1-1/16" and 1-1/8"; I'm pretty sure that I was careful to order the 1-1/8"
At 1/2" of m/c piston stroke that comes out to be .497 in^3 per stroke. That makes it 6 strokes to push the fluid that started in the m/c out of the bleed screw.

What I was alluding to is that I have no way to effectively measure the volume ejected before the bubbles arrive. I use the peanut-butter bleeder to capture the ejected fluid (rather than making a mess) and to insure that no air is drawn back in during the return stroke. FWIW I load the bleed screw threads with dielectric silicone grease whenever I put a new caliper on a vehicle, no air is likely getting in on the return stroke by that means.

Copy, By the way, what is the second black hose off the top if the peanut butter tank going to, just a vent? The copper tube going to below fluid height with black hose made sense since it goes to bleeder screw. Couldn’t figure out the other black hose…

When you swapped MCs with new ones, did each new MC come with a new RPV, or were your swapping the existing RPV to new MC? If swapping it, it does make sense the RPV might have been issue all along.

I think you don’t need to worry about RPV being a pain to bleed (trapping a bubble) given when you bleed you get a good pedal.

 

[ntsqd]

The second black hose on the peanut butter bleeder is either a vent (as I'm using it here) or is connected to intake manifold vacuum when I'm starting with an empty system. Squeezing a Mity-Vac gets really old, really fast and this was a mod that MISF came up with years ago. He connected the Mity-Vac bowl to intake manifold vacuum, I just expended the idea to a container of a useful size. These days I rarely use it with intake vacuum. When working with a new system I now use a Calibration Gas bottle converted into a low pressure bleeder (typically about 10 psi thanks to a low pressure air regulator that a past employer no longer needed) and use air pressure to push brake fluid into the system from the bleed screws.

The m/c's don't come with an RPV. It took some doing just to find one online. I've had to move it to each new m/c. Each of those did supply a new o-ring for the RPV. Can see details of the port in the side of the m/c where the RPV screws in here. As can be seen, it is an ORB type of port, however the threads are not a std ORB port threads (which are the same UNF bolt threads used on AN/JIC fluid fittings). They are very fine for the diameter. If they aren't metric then they could be UNEF (Extra Fine) or they may be something weird. I haven't bothered to measure them.

When the guts of the RPV were removed they left behind a small volume that is above the output hole. This can make bleeding that little air pocket difficult. The system doesn't bleed as fast as it should, it dribbles air bubbles for a long time, and I suspect that this is one of the reasons why. I have used almost 4 quarts of quarts fluid on this system, so far.

 

[Oldtimer]

Nvrstuk has a 2017 thread, at the bottom of this page, where he was having issues with MC volume causing bleeding issues.

 

[ntsqd]

Read it then, and re-read it when it popped up below in the "Similar Threads". Most of the time when someone says "the m/c can't deliver enough fluid" that is really not the problem at all. No one in brake design ever really talks about fluid volume because the caliper pistons do not move very far, so the volumne demand is pretty low. Piston diameter is a far, far more common topic. However, in that thread's case it was true. When overall brake leverage ratios get that high the pedal can feel like the system has air in it even though it doesn't. With such a high leverage ratio the pressure is flexing everything in the system. A real eye-opener for me was to put my hand on a rubber brake hose and then have someone simulate a panic stop.

 

[ksagis]

Read it then, and re-read it when it popped up below in the "Similar Threads". Most of the time when someone says "the m/c can't deliver enough fluid" that is really not the problem at all. No one in brake design ever really talks about fluid volume because the caliper pistons do not move very far, so the volumne demand is pretty low. Piston diameter is a far, far more common topic. However, in that thread's case it was true. When overall brake leverage ratios get that high the pedal can feel like the system has air in it even though it doesn't. With such a high leverage ratio the pressure is flexing everything in the system. A real eye-opener for me was to put my hand on a rubber brake hose and then have someone simulate a panic stop.

Now this an interesting thought. Is it possible that any of your hoses are degraded and somehow the culprit?

I would think they would leak fluid out if they were introducing air in though.

 

[ntsqd]

When I was a tweener kid we had a Saab 96 that had an issue with it's hyd clutch. Mom rebuilt both the m/c and the slave a couple of times and the problem persisted. She thought that she was doing it wrong, so she called grandpa, whom after listening to the exact steps that she had done said that she was rebuilding them correctly. Our next door neighbor came over after something like the third rebuild didn't fix it and suggested that we replace the lone flex hose in the system. (He used to race a Sprint Car at Ascot in LA, he had more than a little mechanical clue.) It was expensive and we didn't have much to spare at all, but mom needed that car to work when she had Call so she bought one and we bled the system, again! It worked. No evidence of it leaking, but it did allow air into the system. That was 40 (?) years ago and I still haven't figured out just how that could be.

All of the hoses on Snowball are braided SS over teflon -3AN hoses with screw together hose ends. I've made a LOT of those hoses over the years, but that doesn't mean that one or more of them isn't a problem. I can tell you that none of them leak fluid. There are three hoses in the rear system, and I bought all of the pieces needed to build and replace each of them. I was literally about to start on the frame to axle hose when I found the small drip coming from the RPV.

 

[Wild horse 75]

I’ve seen rubber hoses do all kinds of weird things. Letting air in. Degrading inside and causing calipers to either not return or slowly return. Then the flex you can get from them causing soft pedal.

 

[nvrstuk]

ALL???? of the hoses are -3AN SS braided hose?

When I was doing my build again, I called and researched at least 6-8 places like Master Power Brakes and they all
unanimously said to use the absolute minimum amount of flex lines possible.

Maybe somewhere in that 30' of line that you are getting the same thing happening that you Mom's car did??? Just a thought.

I ran the bare minimum flex after I contacted all those outfits.

 

[ntsqd]

All of them. Fronts are the same length as the OEM rubber, but with a banjo for the GM front calipers. Rear frame to axle hose is about 19" long (roughly = fronts) because the OEM rubber hose was the droop limiting strap (!!!) as the vehicle came to me. Both rear calipers are sliding type and I debated using a coil of metal tubing instead of a ~6" long hose, but for ease of service and being better tucked out of the way and less likely to snag on brush etc. the hose won.

Once I was part of a crew that built a IMSA Showroom Stock race car in a weekend. Car had been gutted, load-bearing roll cage installed, and painted. Nothing else had been built. ~2 days later the team owner took it for a hot lap around the block of the Sacramento industrial area where his shop was located. The entire brake system plumbing was -3AN SS hose. Not how I would have done it, but it was fast and the pedal feel was surprisingly good.

Were this a race truck or other limited use vehicle I'd have plumbed it from scratch and differently, but it is a long range explorer / desert race support / back-up DD vehicle that I need to have operational. I did try to keep the total hose length to a minimum, but there's only so much you can do with the mostly stock plumbing. Right after bleeding the system the pedal feel is in the top 10 of all of those vehicles that I've ever driven.

 

[nvrstuk]

I'm pretty sure that the outfits I talked to were possibly a little paranoid, however I thought that is a whole lot easier to keep the flex to a minimum and not possibly create a problem.

Sounds like you are good to go... Just gotta find the air source.

 

[ntsqd]

Even -3AN expands under panic stop pressure. Not much, but you can feel it if you're not distracted. They're probably trying not to give people license to do what we did with that IMSA car.

 

[Yeller]

I've done both and used poly like the roundy round guys use. I've had good success with all of it and had good feel. The poly is what really surprised me, the brake feel was good, the longevity was good and it was stupid easy to repair, or disassemble to work on the cars, in a race car or trail car, It's all I would use. Had similar experience with full #3 hose from the master to the calipers, and it is nearly as easy to work with as the poly, just takes more planning. I still prefer Nicopp lines and short hoses for anything that sees the street.

Side note about the poly line. one advantage was if it broke, the only times we had it break it sheared the fitting off of the caliper so I don't think it would have mattered what kind of line/hose we used, a zip tie on the folded over end and we were back in the race with 3 brakes, didn't lose a full minute, didn't refill the master cylinder until we got back to the pits. I have no issues using it. I was a doubter when I got told this is what we are using, no longer a doubter. The cars we used it in had high line pressures, crawlers, with 40"+ tires, 8:1 pedal ratio, 3/4" bore masters, so 1600# of pressure was quite doable and happened.

 

[ntsqd]

I've seen poly used in Karting. Hadn't seen it used on bigger vehicles.

One our old Maint. guys used to do that fold & zip-tie with the poke-n-hope fittings & air lines out on the production floor. Was his default solution rather than a plug for the unused air circuit. And he never trimmed the zip-ties! Drove me nucking futs!

I'm hoping the replacement RPV is actually where USPS says it is because that means that I can remove the old one and start looking at how to modify it. I don't want the replacement RPV for an RPV, I just want a replacement housing in the event that I render this one unusable.

 

[Yeller]

Get that waiting on shipping..... waiting on a single piece of aluminum tubing with fittings on it to complete the AC on the J Truck... it sucks having your DD sitting in the garage. On the bright side, Going to the short course race at Crandon WI this weekend

 

[ntsqd]

USPS says that it's on my front porch. Given the source I'll believe that when I see it sitting there. If it is, then I know what I'm doing with part of my vacation day tomorrow.

I get to install the long delayed PV panels on the pop-top this weekend. I put the electronics in place last Sunday. Not looking forward to poking a hole in the roof. Hope that the tube of Dicor is still usable.

 

[ntsqd]

Well, it was where USPS said it would be, and now it's sitting on my desk.

I removed the old one and opened it up for a thorough look-see. First thing that I noticed is that the seal between the cap and the body of the RPV housing is a cup type seal rather than a square, round, or other type of o-ring, AND that it was installed what I think would be backwards. The cup was facing away from the pressure, not towards it as it would need to be for the pressure to force the seal against the sealing surfaces. I have no memory of disturbing this seal the first time that I pulled this thing apart, but that doesn't mean that I didn't. So I could have put it on backwards w/o realizing it or could have been backwards from the start.

Drew up SW models of the body and cap to get a better idea of just what the internal volume looked like. My Klein digital level said it sits at a 12.8° tilt upwards, so I put them in an assembly at that angle and got this:

Can see the seal oriented as it was when I pulled this apart earlier today.

Made some mods heading in an abandoned direction that I probably would have been better off without, but they aren't looking to be a problem. Looking at the interior shape I concluded that a 'stuffer' tube would be the best option. Will look like this:

The gray part is made from 6061 T6 that I then annealed dead soft so that it will act like a crush washer when the cap is tightened. That's the plan anyway, and it appears to have worked. I set the crush column's length to have a .05" gap where the cap's step meets the end of the body when tightened by fingers only. After tightening it with a wrench as much as I though that I could get away with that gap is about .01" by eyeball. It's back in the m/c and everything is connected. Plan is to bleed it tonight and test drive it.

 

[ksagis]

Well, it was where USPS said it would be, and now it's sitting on my desk.

I removed the old one and opened it up for a thorough look-see. First thing that I noticed is that the seal between the cap and the body of the RPV housing is a cup type seal rather than a square, round, or other type of o-ring, AND that it was installed what I think would be backwards. The cup was facing away from the pressure, not towards it as it would need to be for the pressure to force the seal against the sealing surfaces. I have no memory of disturbing this seal the first time that I pulled this thing apart, but that doesn't mean that I didn't. So I could have put it on backwards w/o realizing it or could have been backwards from the start.

Drew up SW models of the body and cap to get a better idea of just what the internal volume looked like. My Klein digital level said it sits at a 12.8° tilt upwards, so I put them in an assembly at that angle and got this:

Can see the seal oriented as it was when I pulled this apart earlier today.

Made some mods heading in an abandoned direction that I probably would have been better off without, but they aren't looking to be a problem. Looking at the interior shape I concluded that a 'stuffer' tube would be the best option. Will look like this:

The gray part is made from 6061 T6 that I then annealed dead soft so that it will act like a crush washer when the cap is tightened. That's the plan anyway, and it appears to have worked. I set the crush column's length to have a .05" gap where the cap's step meets the end of the body when tightened by fingers only. After tightening it with a wrench as much as I though that I could get away with that gap is about .01" by eyeball. It's back in the m/c and everything is connected. Plan is to bleed it tonight and test drive it.

Nicely done, fingers crossed it works!

Any reason you didn’t go with a copper insert versus Al? If I follow your plan, the seal shouldn’t be wetted at all now and you eliminated the air pocket area.

Still not sure I follow the worry of trapped air pocket remaining after bleed being the issue given you start with a good pedal when you bleed, and slowly get soft pedal over time.

The seal def looks installed backwards for how a typical pressure assisted seal is installed.

 

[ntsqd]

I have aluminum rems of all sorts and I don't have any copper rems. The seal was toast on removal, it's not even in the assembly now. I included it in the model so that y'all could see where it went.

Trapped air pockets just make the bleeding of the system that much more difficult. They're not the source or cause of the current problem.

Didn't get to test drive it last night, but it gravity bled very nicely. Something that it didn't really do well before. I need to pull the head-pipe this evening to look for an exhaust leak, so it will be a couple of days before I can test drive it.