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Old 12-03-2014, 04:10 PM   #113
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OK, you're the guinea pig then?
Been there many times, and I can take it.

Used to work with a guy that had a saying, "Nothing is ever useless, worst case it can be used as a bad example".
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Old 12-03-2014, 04:53 PM   #114
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Disc brake discussion

Concerning the "no appreciable drag" with the 2 PSI RPV, I am with Steve, I think we both make this statement based upon experience using these valves on different vehicles.

I currently own two vehicles that have RPVs that I installed.

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Old 12-03-2014, 05:04 PM   #115
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Originally Posted by J. Morgan View Post
Concerning the "no appreciable drag" with the 2 PSI RPV, I am with Steve, I think we both make this statement based upon experience using these valves on different vehicles.

I currently own two vehicles that have RPVs that I installed.

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I'm not fightin' you.. I have no experience working on anything with the MC lower than the caliper. BUT, I'm not sure the actuator isn't slightly higher than the calipers on the AS.....in my minds eye, as it's not here to really look at.
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Old 12-03-2014, 05:24 PM   #116
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Originally Posted by dznf0g View Post
I'm not fightin' you.. I have no experience working on anything with the MC lower than the caliper. BUT, I'm not sure the actuator isn't slightly higher than the calipers on the AS.....in my minds eye, as it's not here to really look at.
Rich,

My actuator is mounted in the tool box behind the propane bottles, and it's output is about level with the belly of the trailer. If the calipers are below this level, it's not by much.

And then if you figure full braking pressure is 1600 PSI, what is 2 PSI in this setting, something like .0000125% ?

In my mind I'm trying to reduce the time it takes for the actuator to build pressure to the point that braking starts.
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Old 12-03-2014, 05:35 PM   #117
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Here's what is bothering me. 2psi in the line results in 11 pounds of pressure on each pad and 22 pounds of pressure per caliper. On a single piston system, I can see where, after actuator release, normal runouts will push the piston inward in it's bore, overcome the 2psi very briefly and displace a tiny bit of fluid back to the actuator reservoir. You then have zero psi in the line, an appropriate small gap between the pad and rotor, no further drainback (due to the residual valve) and life is good.

With a four piston system, it seems to me that the runouts will just transfer fluid between the caliper halves, not overcome the 2psi "pop-off" and you will always have 22 pounds of pressure (give or take for slight thickness variation around the circumference of the rotor) on the pistons. Maybe the tiniest thickness variation is enough? What am I missing?
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Old 12-03-2014, 05:41 PM   #118
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Rich,

My actuator is mounted in the tool box behind the propane bottles, and it's output is about level with the belly of the trailer. If the calipers are below this level, it's not by much.

And then if you figure full braking pressure is 1600 PSI, what is 2 PSI in this setting, something like .0000125% ?

In my mind I'm trying to reduce the time it takes for the actuator to build pressure to the point that braking starts.
I get that. We are assuming that the system is draining back, pulling the pistons in some and that is why the apply time is lagging, right? Sort of the actuator version of having to depress the brake pedal further down like in the street rod example.

All of that has nothing to do with if and how much a residual 2psi in the line during actuator rest, or just running down the road, affects brake drag.
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Old 12-03-2014, 05:57 PM   #119
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I get that. We are assuming that the system is draining back, pulling the pistons in some and that is why the apply time is lagging, right? Sort of the actuator version of having to depress the brake pedal further down like in the street rod example.

All of that has nothing to do with if and how much a residual 2psi in the line during actuator rest, or just running down the road, affects brake drag.
I suppose we will see.
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Old 12-03-2014, 06:29 PM   #120
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Originally Posted by dznf0g View Post
Here's what is bothering me. 2psi in the line results in 11 pounds of pressure on each pad and 22 pounds of pressure per caliper. On a single piston system, I can see where, after actuator release, normal runouts will push the piston inward in it's bore, overcome the 2psi very briefly and displace a tiny bit of fluid back to the actuator reservoir. You then have zero psi in the line, an appropriate small gap between the pad and rotor, no further drainback (due to the residual valve) and life is good.

With a four piston system, it seems to me that the runouts will just transfer fluid between the caliper halves, not overcome the 2psi "pop-off" and you will always have 22 pounds of pressure (give or take for slight thickness variation around the circumference of the rotor) on the pistons. Maybe the tiniest thickness variation is enough? What am I missing?
OK, I re-read your post above and I don't understand it. I don't understand how you have computed that 2PSI in the line computes to 11 pounds on each pad. In my thinking, pressure is pressure anywhere in the system, kind of like kinetic energy, or voltage on a wire. Measure the voltage any where on the wire, assuming no high current, the voltage is the same.

I would also think that 2PSI applied to a hydraulic system would be 2PSI anywhere in the system. No?
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Old 12-03-2014, 06:30 PM   #121
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Steve, have you tried activating the actuator without pressing brakes? That would tell u if there is a "lag" due to actuator spinup or a delay from the TV brake signal.
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Old 12-03-2014, 06:40 PM   #122
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2 psi anywhere in the system. correct. 11 lbs force from the piston figuring its area.
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Old 12-03-2014, 07:02 PM   #123
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Steve,

If I may, what problem are you presenting and hoping to improve? Just for clarification.

Gary

Bleeding the Dexter 4 piston/caliper system.

I wish I had a simple easy bleeding process for the 4 piston Dexter system as supplied and configured by Airstream. I don't have one. I do have the modification, technique and video proof of PSI at the caliper vs. time. This was presented in the link to my older post. I also wish I had an easy way to present how much of my current professional activities are focused on helping our customers understand and correctly bleed their clutch hydraulic release systems. Here's my philosophical answer below.

Create a visual representation of the hydraulic system, focus on the fluid flow path and WHERE the high points in the system are. Those high points are your focus points, the air hides in them, how do you get them out? The inboard caliper half is a prime air bubble trap example.

Commonly used techniques:

Vacuum bleeding at a bleed screw (Miti Vac and others) is a very useful tool but it CANNOT pull a trapped air bubble down from a air trap like the inboard caliper half of the Dexter piston.

Reverse Fluid Injection (RFI) or pressure bleeding via the outboard caliper bleed screw has the same challenge, pushing the air bubble down and out of the outboard caliper.

Dry Vacuum bleeding at the actuator fill cap. This technique can pull an air bubble up, but my testing found that it cannot pull a bubble DOWN to get it to go up and out. Dry Vacuum bleeding is recognized by Ford and GM for bleeding both as factory service procedures and published as TSB's for clutch hydraulic release system servicing.

Pressure bleeding via the actuator fill cap, we go right back to trapped air in the inboard caliper.

Bench bleeding. If the line connection had a bleed screw or line connection that could be "cracked" you might gain something from trying to fill the caliper halves, but IMHO you have no chance of this working at all because of the flared hose end to flared hose end line from the "T" or 90 to the caliper, you have to spin the the caliper half onto the hose fitting.

Bleeding the inboard caliper in some technique while bleeding the system is a requirement. My objection to the system as plumbed by Airstream is the absence of a line connection that can be loosened/cracked for bleeding and successfully re-tightened to position w/o creating an undesirable twist to the hose.

An air bubble WILL delay the creation of pressure. Pressure is required for braking. The electric actuator will continue to pump and compress the bubble completely and then finally create PSI, but that is a source of delay. that's why I made a bleed screw gauge adapter and video'd the gauge using the actuator audio track to determine Time vs. PSI.
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Old 12-03-2014, 07:48 PM   #124
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If you guys are trying to make me feel happy about having drum brakes, you're doing a great job.

Thanks,
Ken
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Old 12-03-2014, 08:15 PM   #125
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Oh, Ken, we could start ripping them too, but if you're happy, I wouldn't want to bring you down.
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Old 12-03-2014, 08:29 PM   #126
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OK, I re-read your post above and I don't understand it. I don't understand how you have computed that 2PSI in the line computes to 11 pounds on each pad. In my thinking, pressure is pressure anywhere in the system, kind of like kinetic energy, or voltage on a wire. Measure the voltage any where on the wire, assuming no high current, the voltage is the same.

I would also think that 2PSI applied to a hydraulic system would be 2PSI anywhere in the system. No?
2psi X pi X radius squared

Pistons are 1.875" in diameter or .9375 radius

2psi X 3.14159 X .8789 = 5.52 pounds of force per piston X 2 pistons per pad = 11.04 pounds of force per pad X 2 pads per rotor = 22.08 pounds of total force applied per rotor.
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