2 years chasing low pressure at the caliper...
- Thread starter Icetech
- Start date
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I don't know. I remember though Ford actually had a TSB on it. Said to do the same thing we tried and put a non-directional finish in the rotors and it worked. Ford "said" it had something to do with the rotors being composite rotors, but there were plenty of other composite rotors out there at the time, so I never bought that.
goldduster318
Overzealous Car Modifier
Pressure doesn't change in a closed hydraulic system due to a restriction. The rate that pressure increases or decreases does.
When your pressure readings are different, does this mean you have removed the calipers/lines from the system on your Master Cylinder measurements?
The Master Cylinder by itself can do pretty much infinite pressure if you can do enough of an input and don't bottom out the pistons first.
When you bench bleed, do you push the close to the bottom and work it back and fourth and then do near the beginning of the stroke? You will get the ports of the two circuits closing at different travels when they aren't bled because master cylinders have a spring bias.
The rear brakes not working correctly will also make it much harder to have the fronts working correctly due to the same bias. Especially with smaller master cylinders the piston travel can get eaten up pretty fast with a disk brake and anything going on with the rears, including too much shoe clearance. Adjust the shoes to the point in which you can just barely get the drum on if they are not already. 7/8 MC should have a lot of travel and not be that far off the floor with an intact system, as I ran a stock disk/drum with 15/16" at one point and that had a lot of travel as well.
When the caliper is in the system you have to displace more fluid in order to build the pressure, so of course the input force will be more because there is flex in the system, especially in the caliper bridge.
When your pressure readings are different, does this mean you have removed the calipers/lines from the system on your Master Cylinder measurements?
The Master Cylinder by itself can do pretty much infinite pressure if you can do enough of an input and don't bottom out the pistons first.
When you bench bleed, do you push the close to the bottom and work it back and fourth and then do near the beginning of the stroke? You will get the ports of the two circuits closing at different travels when they aren't bled because master cylinders have a spring bias.
The rear brakes not working correctly will also make it much harder to have the fronts working correctly due to the same bias. Especially with smaller master cylinders the piston travel can get eaten up pretty fast with a disk brake and anything going on with the rears, including too much shoe clearance. Adjust the shoes to the point in which you can just barely get the drum on if they are not already. 7/8 MC should have a lot of travel and not be that far off the floor with an intact system, as I ran a stock disk/drum with 15/16" at one point and that had a lot of travel as well.
When the caliper is in the system you have to displace more fluid in order to build the pressure, so of course the input force will be more because there is flex in the system, especially in the caliper bridge.
Firstblood
Well-Known Member
The straw color is hot, the blue/purple is real hot!
Basically tempered the metal.
That is what I have said. That means the master cylinder is not developing the pressure.
That's an odd looking MC.
I'd put on a factory style one before I got too bent out on other stuff.
If one system fails, you should get braking from the other until the pedal hits the floor.
Pump the brakes.
The open failed system will continue to allow the pedal to go to the floor, though.
Especially if it's the front, which will also dramatically increase the stopping effort of the now rear only, and compromised system.
There's a LOT of changed out stuff, that's far from factory in the OP's system.
I'd put on a factory style one before I got too bent out on other stuff.
If one system fails, you should get braking from the other until the pedal hits the floor.
Pump the brakes.
The open failed system will continue to allow the pedal to go to the floor, though.
Especially if it's the front, which will also dramatically increase the stopping effort of the now rear only, and compromised system.
There's a LOT of changed out stuff, that's far from factory in the OP's system.
goldduster318
Overzealous Car Modifier
Other thing to note is DOT5 has higher compressibility than DOT3/4. You have to work harder to get the same fluid pressure.
Icetech
Well-Known Member
that shouldn't make it drop 450psi in 10" of hose...
Icetech
Well-Known Member
the MC makes 1000+ PSI tested at the MC and down to just after the distro block.
The only way the pressure could drop is if there was flow across an orifice (restriction).
Icetech
Well-Known Member
yes.. that's what i'm trying to find... a new flex line will be here tomorrow night..
BTW.. @YY1 i have had 4 mcs on it... 2 stock iron ones a drdiff one, and this 7/8 which is a mopar one also.. if a master cyl makes pressure it should be able to pressurize the system if it's numbers correct or not
P.S. don't read any of that as bitchy.. long day and i'm hungry
goldduster318
Overzealous Car Modifier
Pressure is the same everywhere in a closed hydraulic system. That's not what's happening here.
When you take compliance out of the system by removing the caliper or hose you're just removing displaced volume.
That IMO junk DOT5 is really prone to bubbles forming and being hard to remove. You have to work ALL of them out which may be really challenging. DOT3/4, you can fill an empty system, bleed once, wait overnight, and bleed a second time and have it perfect no matter how much of an air trap there is there.
Only thing I can tell you is force the brakes on as hard as possible and wedge them, wait overnight and then bleed again.
With a time period more than a few milliseconds, that's not true either. It's a flow restriction. Hydraulic pressure equalizes.
Icetech
Well-Known Member
yeah, next time i test i will hold it...
I knew bout the dot5 thing, i have actually bled it 10+ times with days between so it could settle then work out any air...
i have no issue with dot3, but i don't feel like replacing the entire system yet again if i can get this working.. other people use dot5 without a problem.. and i like not destroying paint
I converted to DOT 5 in 2016.
It does take a slightly higher push of the foot and the pedal travels just a bit further, but not enough to affect braking at all.
Icetech
Well-Known Member
yeah, i have seen guys talk bout running it for 20 years without issue..
This problem is not like anything i have seen before.. my cobra even had manual discs and they just worked.. i think that even just used a fox body power master cyl too.. (the car was built using a fox donor)
at this point the only thing i haven't changed is the distro block.. if the hose tomorrow does the same thing (which i think it will) i'm gonna pull the block and check it over well..
nice thing bout dot5 is being non corrosive.. just wipe up and no sweat
Icetech
Well-Known Member
yeah, guys say when you agitate dot5 it gets micro bubbles bad and due to its viscosity it takes time for them to reabsorb so you can move them when bleeding..
i have been reading/fighting with this for a long time
Totally unrelated but maybe comic relief
Decades ago, I was working on a computer that came in because the keyboard didn't work.
Put a new one on- still didn't work.
Only one other possibility- motherboard.
Put in a new MLB- still no keyboard.
Put on a second keyboard- still no KB.
WTF
Turns out part of the memory "handler" used to go through the keyboard.
Not sure how it's related, but what was happening was the MLB was blowing the KB, and then the bad KB was popping the new MLB.....and so on and so on.
New MLB and new KB at the same time and bad stuff in the trash TODAY.
This is the kind of shite that happens to me all the time.
Does make you think of "what else could possibly happen".
Decades ago, I was working on a computer that came in because the keyboard didn't work.
Put a new one on- still didn't work.
Only one other possibility- motherboard.
Put in a new MLB- still no keyboard.
Put on a second keyboard- still no KB.
WTF
Turns out part of the memory "handler" used to go through the keyboard.
Not sure how it's related, but what was happening was the MLB was blowing the KB, and then the bad KB was popping the new MLB.....and so on and so on.
New MLB and new KB at the same time and bad stuff in the trash TODAY.
This is the kind of shite that happens to me all the time.
Does make you think of "what else could possibly happen".
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