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Posessed hydraulics

H-K

H-K

INFIDEL
Supporting Member
Location
SLC UT, USA
Sigh :rolleyes:
I'm having a problem with my brake hydraulics and would appreciate help if possible. I've searched around and haven't found anything specifically applicable to my particular problem and I'm running out of things to try.

Background:
I have a '90 YJ (power brakes) and recently swapped the axles with a D44 front and a Ford 9" in the rear. Front brakes are new Chev calipers and the rear are new Ford wheel cylinders.

Problem:
Before the axle swap, the master cylinder was in fine working order. After swapping the axles, the brake pedal was soft as expected, but would also occasionally lose resistance and go to the floor. The brakes would pump back up, and be fine for a while and then suddenly, randomly drop pressure and sink to the floor. This indicated to me that the master cylinder had gone bad, coincidentally, at some point during the axle swap. So, I replaced it with a rebuilt one and had the exact same problem. I figured it was just a defective reman and so again, I decided it was just another coincidence and took it back for a replacement. After installing the replacement, I'm still having the problem.

At this point I'm thinking that the odds are pretty low that my original MC spontaneously failed along with two more MCs. Based on my limited understanding of brake hydraulics, the symptoms scream "BAD MASTER CYLINDER" -- I mean I can't think of what else would cause it. Air in the lines would result in a consistent, albeit low, level of pressure, so I don't think it could be just a bleeding issue. The only other thing I can think of is some sort of an intermittent leak that results in a sudden loss of pressure. However, I've checked the parts and there is no leakage.

Any help would be appreciated. thanks :)
 
H-K

H-K

INFIDEL
Supporting Member
Location
SLC UT, USA
James K said:
I would lean towards air still being in the system somewhere :-\
Click to expand...

hmmmm.
Would you expect the pressure to be consistent though? The pressure seems to be soft, but consistent and then suddenly just goes to zero. it's consistent about 95% of the time.

thanks
 
Brett

Brett

Meat-Hippy
Location
Salt Lake City Utah
My friend had the same problem when he put his front 44 in and it turned out that there was air still in the calipers. What he had to do was unbolt them from the axle and rotate them around while tapping them with a hammer to get the air out. I'd give that a try and maybe have someone helping you bleed them out at the same time.
 
H-K

H-K

INFIDEL
Supporting Member
Location
SLC UT, USA
Brett said:
My friend had the same problem when he put his front 44 in and it turned out that there was air still in the calipers. What he had to do was unbolt them from the axle and rotate them around while tapping them with a hammer to get the air out. I'd give that a try and maybe have someone helping you bleed them out at the same time.
Click to expand...

Were his brakes just soft or did they occasionally drop completely in pressure? I'm having a hard time seeing how air in the lines would be so vastly and suddenly inconsistent in pressure.
thanks
 
James K

James K

NO, I'm always like this
Location
Taylorsville, Ut
H-K said:
hmmmm.
Would you expect the pressure to be consistent though? The pressure seems to be soft, but consistent and then suddenly just goes to zero. it's consistent about 95% of the time.

thanks
Click to expand...



air in the lines is like having a leak on it is internal and can not be seen.

do like Brett suggested.
 
B

bobdog

4x4 Addict!
Location
Sandy
In theory air in the lines should give consistant results but who knows. I would start with new rear wheel cylinders they are one of the things that changed when the problem started. They are so cheap that it can't hurt to try. If that does not solve your problem then move on to the fronts.
 
H-K

H-K

INFIDEL
Supporting Member
Location
SLC UT, USA
James K said:
air in the lines is like having a leak on it is internal and can not be seen.

do like Brett suggested.
Click to expand...

Okay, I'll go do it a third time, but just to clarify the question, wouldn't air in the lines just make them consistently soft? Can air in the lines cause a sudden drop from about 50% pressure to 0?

thanks
 
James K

James K

NO, I'm always like this
Location
Taylorsville, Ut
H-K said:
Okay, I'll go do it a third time, but just to clarify the question, wouldn't air in the lines just make them consistently soft? Can air in the lines cause a sudden drop from about 50% pressure to 0?

thanks
Click to expand...


maybe/maybe not.


the air will move around in the lines and it is esssentially a soft stop in the line, thus it will compress and creat the soft to zero peddle feel.
 
H-K

H-K

INFIDEL
Supporting Member
Location
SLC UT, USA
James K said:
maybe/maybe not.


the air will move around in the lines and it is esssentially a soft stop in the line, thus it will compress and creat the soft to zero peddle feel.
Click to expand...

Well, since I'm gonna be re-bleeding everything, I guess I'll just take the opportunity to do the E350 Master cylinder swap.
 
B

bobdog

4x4 Addict!
Location
Sandy
James K said:
maybe/maybe not.


the air will move around in the lines and it is esssentially a soft stop in the line, thus it will compress and creat the soft to zero peddle feel.
Click to expand...

It does not mater if it moves around or not. It is part of the closed system and is subject to Pascals Law. It will compress every time the same amount every time no matter where it is in the lines or calipers, ect resulting in the same loss of hydrolic pressure every time.
 
Caleb

Caleb

Well-Known Member
Location
Riverton
well, IMO, like bobdog is stating, air shouldn't cause a loss of pedal (but air in the lines does weird things sometimes)...I would crawl around your brake system and see if there is leaking anywhere...if you can find no leaking then you would have to assume its internal, being your MC was good before the swap it would be my last suspect. In that case air would be my #1 suspect. BTW, I ran a D44 from a waggy and a 9" from an EB in my YJ and once the brakes were bled good I could lock up all four corners with my stock MC and no prop valve...
 
C

chevtech

Seasoned Mall Cruiser
Location
Next door
bobdog said:
It does not mater if it moves around or not. It is part of the closed system and is subject to Pascals Law. It will compress every time the same amount every time no matter where it is in the lines or calipers, ect resulting in the same loss of hydrolic pressure every time.
Click to expand...

Rear ABS?!?!?

Check and see for sure. If so I probably know your problem.
 
C

chevtech

Seasoned Mall Cruiser
Location
Next door
It could possibly be air in the system though if there is so much that it causes the valve in the combo valve (I forget what it is called specifically) to slam shut preventing pressure from being lost to both ends when you lose pressure on one circuit. Maybe it is sticking closed sometimes and then popping open.

Do your frt and rear brakes actually work?
 
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