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3/4 ton brake and MC issue

98CherokeeSport

NAXJA Forum User
Location
Bonney Lake, WA
so i have read and read and read on NAXJA and Pirate4x4 about brakes. i just swapped in full width dana 44 and dana 60 into my xj. the front is running the dual piston 3/4 ton brakes and the rear has the 3/4 ton k20 disc brake setup. my xj is a 1998 so it has the dual diaphram booster. i read that the 1 ton dodge 3500 MC would be what i need to bolt right up to the booster. so i bought that and made a little 5/8ths spacer like i read about, bench bled it and then installed it and bled the brakes. when the engine was off i got really good pedal pressure. but as soon as i started it up i lost almost all of the pressure. i took it for a drive around the neighborhood and i could stop but i had to put it almost al the way to the floor and the pedal was really soft. o and i have the stock prop valve. does anyone have an idea of what my issue could be or some solutions? thanks!
 
i have put about 3 quarts through the whole system already and made sure i bench bled the master really well. i had a little play in the pedal before the push rod would hit the MC so i made my spacer a little bit bigger. this helped out some. i now feel somewhat comfortable driving it on the road but i know it can get better. any other ideas?
 
. i had a little play in the pedal before the push rod would hit the MC so i made my spacer a little bit bigger. this helped out some.

maybe its just too late for me to comprehend this, but isn't this a little backwards? If there's freeplay between the pushrod and the master, wouldn't spacing the master farther away from the booster create even more freeplay? Did your MC come new in a box? Are you using the dodge prop valve?
 
[FONT=&quot]
. i took it for a drive around the neighborhood and i could stop but i had to put it almost al the way to the floor and the pedal was really soft. o and i have the stock prop valve. does anyone have an idea of what my issue could be or some solutions? thanks!

When you went to the 3/4 ton brake, you got bigger caliper pistons, great for more power but they require more fluid volume to push to bigger pistons.
I do not know what year Dodge 3500 MC you bought. I checked Rock auto for a 2005 Dodge 3500 and found they come with a 15/16" cylinder.
Other year models use different size MCs.

XJs use a 1" cylinder. So what you may have done was to reduce volume instead of increasing it.
The Dodge 15/16”bore MC gives better pressure but less volume and that is why your pedal almost goes to the floor.
I would first try the stock XJ cylinder and then go the Dodge Dakota's MC (1.125" bore) if there is still not enough volume (pedal height)

Here some photos of usable cylinders.
http://www.flickr.com/photos/lwade/sets/72157616348032595/[/FONT]
 
I would also compare the size of your hard lines to what came with the axles - are your lines flowing enough volume?
Disregard this if its dumb, but makes sense to me...
 
I would also compare the size of your hard lines to what came with the axles - are your lines flowing enough volume?
Disregard this if its dumb, but makes sense to me...

The stock hard-lines are more than adequate as long as they are in good shape.
 
maybe its just too late for me to comprehend this, but isn't this a little backwards? If there's freeplay between the pushrod and the master, wouldn't spacing the master farther away from the booster create even more freeplay? Did your MC come new in a box? Are you using the dodge prop valve?

im talking about a spacer in the master cylinder that the push rod goes into. its deeper than a cherokee one so it needs a spacer for the push rod to actually push the cylinder. yes it was brand new and i am using my stock prop valve

Your rear calipers aren't on upside-down are they? If the bleeder isn't on top you will never get the air out. Don't ask how I know this can cause the symptoms you are having... :doh:

they are all facing up. i did the same thing a while back :laugh:

[FONT=&quot]When you went to the 3/4 ton brake, you got bigger caliper pistons, great for more power but they require more fluid volume to push to bigger pistons.[/FONT]
[FONT=&quot]I do not know what year Dodge 3500 MC you bought. I checked Rock auto for a 2005 Dodge 3500 and found they come with a 15/16" cylinder. [/FONT]
[FONT=&quot]Other year models use different size MCs.[/FONT]

[FONT=&quot]XJs use a 1" cylinder. So what you may have done was to reduce volume instead of increasing it.[/FONT]
[FONT=&quot]The Dodge 15/16”bore MC gives better pressure but less volume and that is why your pedal almost goes to the floor.[/FONT]
[FONT=&quot]I would first try the stock XJ cylinder and then go the Dodge Dakota's MC (1.125" bore) if there is still not enough volume (pedal height)[/FONT]

[FONT=&quot]Here some photos of usable cylinders.[/FONT]
[FONT=&quot]http://www.flickr.com/photos/lwade/sets/72157616348032595/[/FONT]

mine is out of a 1998 3500. i heard thats the one to use.

so i took it for a drive today and as i kept driving the pedal seemed to get a little bit stiffer. not perfect but an improvement
 
im talking about a spacer in the master cylinder that the push rod goes into. its deeper than a cherokee one so it needs a spacer for the push rod to actually push the cylinder. yes it was brand new and i am using my stock prop valve

yeah, i realized that must have been what you were talking about after i posted. Maybe one day ill learn not to post half drunk and past midnight:banghead: If your noticing an improvement after just driving around the only reasonable explanation seems to be a few stubborn air bubbles left in the system. Are the rear calipers clocked at an angle that may make them hard or impossible to properly bleed? Maybe pick up a vacuum bleeder and see if you can work em out like with that. Sometimes a more constant flow of fluid can help dislodge bubbles from calipers and so on.
 
Probably a stupid question but will doing this swap help with stock front calipers and explorer discs in the rear? Not saying I need it but just wondering. Not sure if it would be to much volume to these calipers or if it would actually help braking.
 
Wish I read this thread two months ago....... Although when the bleeders are facing down they pour right into a catch pan. My brakes work now!!!!!!
 
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