NLXTACY
Wits' End
I know right? Sheesh.
I'm at my absolute wits end- Fading Brake pedal despite best efforts (1 Viewer)
- Thread starter musthave
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I know right? Sheesh.
NLXTACY
Wits' End
FWIW, if you have the where-with-all to change everything then by all means change it all. These trucks are old and very very very few of you have the advantage of owning since day one to know what's been changed. So why not replace everything at once? It means not having to be concerned about it again for another 200k. And it's not like it's a new motor. It's just brake components.
Uh... Never said anything was wrong. I am intrigued by the problem though. It seems like these guys have been going after it pretty hard for a while now. Wondering if they have anything new. And definitely curious how it will end... if it does.What's wrong with that?
Sweet, take some pics of how you measure and adjust the booster push rod!
TrickyT
Hate that mud...
Here's a schematic diagram I found in my collection of files that shows the entire FZJ-80 braking system. I don't remember what 'mud thread this originally came from, but the schematic matches the physical fluid circuit layout that was posted several pages back. What's nice about this one is that it shows how the ABS pump fits into the fluid circuit. It's also the first document I remember seeing that calls the solenoids a "3-position solenoid" and not 3 separate solenoids. Previously I had jumped to the conclusion that there were 3 different solenoids in the ABS actuator. And back in post #85 Pin_Head suggests grounding pins 1, 3 and 4 on the 6 position plug in order to activate the all solenoid valves at once. But now I'm thinking this isn't the right way to exercise the ABS actuator. Instead what you need to do is ground those three solenoid connector pins one at a time and also activate the ABS pump for a few seconds during and after when each solenoid circuit is actuated. Doing this will pump brake fluid through all the internal fluid circuits of the ABS.
Just to be clear, the schematic diagram shows 3 separate fluid circuits for the 3-position solenoid. So you need to activate each circuit (LF, RF, Rear) separately in order to exercise all 3 fluid circuits. Maybe, somehow, you've still got air trapped in one or more of these circuits or the two internal reservoirs that are shown. Obviously I'm just guessing here, but it's a fresh idea to what otherwise seems a vexing problem.
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Nice diagram. I haven't seen that one before.
It looks like what they call a "3 position valve" is really a 3 port (inlet, outlet1, outlet2) 2 state solenoid valve. When energized, it switches to outlet 2 which routes the fluid to a spring loaded buffer (reservoir) and then to the pump, which gives the characteristic pulsating pressure delivered to the caliper through a one way check valve. I presume they are check valves (7 total) due to their proximiaty to the inlet and outlet of the pump, which are necessary to provide directionality. I am not sure which direction the pump flows, but maybe the arroheads indicate it flows from bottom to top.
My guess is that it doesn't matter if you energize the valves one at a time or all 3 at once because it looks like you get the same results, but it is easy enough to test.
It is not obvious how you would get air in there in the first place. It seems to me that one or more of the solenoid valves or check valves would have to leak. If they leak, that alone could explain the spongy pedal / dead volume if the volume of the "reservoir" is large enough. If you feel like swapping more parts, you might as well swap in another ABS unit. In for a dime, in for a dollar.
You could also try cleaning out the valves. A ultrasonic cleaner would be appropriate.
It looks like what they call a "3 position valve" is really a 3 port (inlet, outlet1, outlet2) 2 state solenoid valve. When energized, it switches to outlet 2 which routes the fluid to a spring loaded buffer (reservoir) and then to the pump, which gives the characteristic pulsating pressure delivered to the caliper through a one way check valve. I presume they are check valves (7 total) due to their proximiaty to the inlet and outlet of the pump, which are necessary to provide directionality. I am not sure which direction the pump flows, but maybe the arroheads indicate it flows from bottom to top.
My guess is that it doesn't matter if you energize the valves one at a time or all 3 at once because it looks like you get the same results, but it is easy enough to test.
It is not obvious how you would get air in there in the first place. It seems to me that one or more of the solenoid valves or check valves would have to leak. If they leak, that alone could explain the spongy pedal / dead volume if the volume of the "reservoir" is large enough. If you feel like swapping more parts, you might as well swap in another ABS unit. In for a dime, in for a dollar.
You could also try cleaning out the valves. A ultrasonic cleaner would be appropriate.
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^^ sounds right ^^
I think that might be what was happening, and I know we were modulating all 3 when they went to ground together.
I also think the ABS is fine, and that the reservoir was holding aged or older fluid, but didn't contribute to the dis-function that WAS the problem here.
I'll leave it for Tim to explain, but he has brakes, finally.
I think that might be what was happening, and I know we were modulating all 3 when they went to ground together.
I also think the ABS is fine, and that the reservoir was holding aged or older fluid, but didn't contribute to the dis-function that WAS the problem here.
I'll leave it for Tim to explain, but he has brakes, finally.
reconranger
SILVER Star
Darn tooting it's time for a 6-4-6.
I forgot which is which.
I forgot which is which.
So..... what finally fixed it? I'm at my absolute wits end waiting for the answer!
Subscribed.
- Thread starter
- #233
How did we measure the pushrod?
We measured the distances and differences in depth from the end of the master cylinder to where the pushrod touches on the piston inside of the master cylinder.
We then adjusted the pushrod as needed.
We measured the distances and differences in depth from the end of the master cylinder to where the pushrod touches on the piston inside of the master cylinder.
We then adjusted the pushrod as needed.
- Thread starter
- #234
The PO installed in aftermarket brake booster. What is interesting is where the vacuum goes in it does not have the same attachment as the one pictured below.
If you notice just to the left of the vacuum hose and to the right of the master cylinder against the booster you can see that it is secured with a bracket.
That bracket does not exist on my booster. So this part of the booster moves freely.
I'm not sure if this can effect any performance of the brake booster.
Chesters
Mine
If you notice just to the left of the vacuum hose and to the right of the master cylinder against the booster you can see that it is secured with a bracket.
That bracket does not exist on my booster. So this part of the booster moves freely.
I'm not sure if this can effect any performance of the brake booster.
Chesters
Mine
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- Thread starter
- #236
The resolution was a master cylinder. There is still some work to be done but at least I have brakes now.
By no means do I regret all of the other parts that were purchased as they were all necessary.
I essentially have an almost entirely new braking system every component except the ABS actuator.
By no means do I regret all of the other parts that were purchased as they were all necessary.
I essentially have an almost entirely new braking system every component except the ABS actuator.
- Thread starter
- #237
This is a HUGE help in understanding how things work, great find!!!
- Thread starter
- #238
LOL, yea, I was curious too! When you work on brakes so long, then you test them, and the question, "How do they feel" comes up. There is never a good answer, hell, they are different than they were, but better, IDK. I got so used to crappy brakes that I couldn't tell crappy from really crappy.
The experience gained is priceless.
It's not ended, still need to bleed the system again, and get any last bit of air out. Will test the ABS again using our new jig, just to be sure no air at all in that. Might adjust the pedal a bit.
I just ran the green 80 around, heated the brakes up a bit, forced the ABS to modulate, no difference from what I can tell. My booster seems to have longer lasting vacuum assist capacity than yours, though I can still exceed it at idle with multiple depressions of the pedal. I think I'll put together a bracket stay we can install on yours, see if that holds back the leakage through that valve a bit.
