Brakes are crap after rear disc swap!!

I did a rear disc swap for a friend on a 14bolt using 1/2 ton chevy calipers and installed a 86 e350 master, took the prop. valve out, and bled the crap out of it and the pedal still goes pretty much to the floor. I capped the rear line at the master cyl. and the pedal now feels real good. My question is could the stock 3/16 brake line running to the back possibly not allowing enough flow to the rear? If so would a 1/4 line be enough? If not does anybody have any other ideas what could be causing this?

Thanks in advance!!
Jake

Refresh my memory - does the 86 E350 m/cyl have a rear circuit for disc brakes, or just for drums?

And, have you done anything to the proportioning valve? I haven't done this swap yet, but I've got some decent experience with hydraulics, so I'm not a compleat neophyte. I do know that the disc calipers require more fluid than the drum brake wheel cylinders - so your flow question may be correct (but for the wrong reason.)

Removing the proportioning valve (I just re-read your post) won't compensate for the master cylinder not supplying enough fluid volume for the rears in the first place...

I wouldn't think having too small of a line would not make the pedal go to the floor, I would think it would be too stiff. Also I doubt the 1/2 ton brakes require larger lines, IIRC my F150 takes the same size as my Jeep.

Do you have your calipers mounted correctly? The bleeder valve needs to be towards the top or you will never be able to get all the air out.

Otherwise it could be the master cylinder, like 5-90, I'm not sure if it has enough volume for the 1/2 ton calipers.

I had this problem in my camaro. Did you bleed the master cylinder real well? Mine had just a little air in it and did exactly what your describing. After bleeding it again while in the car it worked perfect.

What those guys said. I'm guessing that the rear disks require more fluid volume than the drum setup and you need a bigger bore master cylinder. The piston in the master cylinder cannot pump enough fluid to the rear to move the caliper pistons far enough.

Two guesses: One, air still in the system, and two, wrong MC. This is what you want for a Cherokee running a single diaphragm booster: http://napaonline.com/masterpages/N...603&Description=Brake+Master+Cylinder+-+Remfd

The 3/16th lines are fine. Get the '76 Mercury Grand Marques 1.125 MC, bench bleed properly, insure push rod properly adjusted, and you should be "good to go".

important question what year XJ? If its 97+ then the prop valve and booster should be sufficient.

5-90 said:

Refresh my memory - does the 86 E350 m/cyl have a rear circuit for disc brakes, or just for drums?

And, have you done anything to the proportioning valve? I haven't done this swap yet, but I've got some decent experience with hydraulics, so I'm not a compleat neophyte. I do know that the disc calipers require more fluid than the drum brake wheel cylinders - so your flow question may be correct (but for the wrong reason.)

Removing the proportioning valve (I just re-read your post) won't compensate for the master cylinder not supplying enough fluid volume for the rears in the first place...

Click to expand...

The 86 e350 is just for drum brakes but it has a 1.25 piston bore which from what I have read is good enough for rear disks. When people do rear disc swaps I've seen lots of people reuse the stock master and it works fine so jumping up to a master with that much bigger of a bore should work great should it not?

Do you think reinstalling the prop. valve would help at all? I just assumed that without one the brakes should have no trouble working and would prolly work too well if anything. Maybe that is wrong to think that.

TRAILREADYXJ said:

I wouldn't think having too small of a line would not make the pedal go to the floor, I would think it would be too stiff. Also I doubt the 1/2 ton brakes require larger lines, IIRC my F150 takes the same size as my Jeep.

Do you have your calipers mounted correctly? The bleeder valve needs to be towards the top or you will never be able to get all the air out.

Otherwise it could be the master cylinder, like 5-90, I'm not sure if it has enough volume for the 1/2 ton calipers.

Click to expand...

I just thought maybe without a large enough line maybe there wasn't enough volume to push the caliper piston back out completly after it has retracted so it doesn't drag.

The calipers are mounted correctly. That is definetly a good question cause a guy I know couldn't get his to bleed after trying for a long time and it ended up he had them installed upside down:banghead:

CORRECT I MIGHT BE WRONG
Pressure= force / Area
larger bore = less pressure to the rear ??
With the force being constant

jeepme said:

I had this problem in my camaro. Did you bleed the master cylinder real well? Mine had just a little air in it and did exactly what your describing. After bleeding it again while in the car it worked perfect.

Click to expand...

I've ran over a quart of brake fluid through the system and also it was bench bled prior to installing it. I wouldn't think there is any air left but I will definetly bleed it some more.

Why use a master cylinder intended for drum rears, when an easier swap is to use one intended for 4-wheel discs? If you're talking early XJ (single diaphragm booster), the MC I referenced above is for you. I converted to rear discs, using the Caddy metric calipers, which have a smaller bore than the Chevy 1/2-tons. I found the stock MC inadequate. I swapped in 1.125 MC intended for rear drums, and that one was barely adequate. I then swapped in the Mercury MC, and it's adequate with my new front 1/2-ton calipers, which are larger than stock XJ fronts; if I could find an MC that had a little larger bore, that would fit my current stock single diaphragm booster, I'd swap again.
Any commercial disc brake-kit manufacture will tell you there is a difference in an MC intended for drum rears versus one designed for disc rears, and it's more than just the reservoir size.
Removing your prop valve insures it's not a problem. The only reason you may need one, is the rear brakes may lock up before the fronts. If that happens, then a prop valve is needed to reduce rear brake response.

TRAILREADYXJ said:

Do you have your calipers mounted correctly? The bleeder valve needs to be towards the top or you will never be able to get all the air out.

Click to expand...

Did you answer this question?

xjbubba said:

Two guesses: One, air still in the system, and two, wrong MC. This is what you want for a Cherokee running a single diaphragm booster: http://napaonline.com/masterpages/NOLMaster.aspx?PageId=470&LineCode=TS&PartNumber=101603&Description=Brake+Master+Cylinder+-+Remfd

The 3/16th lines are fine. Get the '76 Mercury Grand Marques 1.125 MC, bench bleed properly, insure push rod properly adjusted, and you should be "good to go".

Click to expand...

Maybe the e350 master isn't enough but I know I've read in more than a few threads that it was a popular upgrade and worked well. I've already purchased this reman master and modified it to fit so I would like to see it work but if there is no way that it will I'll have to get the merc one and do that. Thanks

xjbubba said:

Why use a master cylinder intended for drum rears, when an easier swap is to use one intended for 4-wheel discs? If you're talking early XJ (single diaphragm booster), the MC I referenced above is for you. I converted to rear discs, using the Caddy metric calipers, which have a smaller bore than the Chevy 1/2-tons. I found the stock MC inadequate. I swapped in 1.125 MC intended for rear drums, and that one was barely adequate. I then swapped in the Mercury MC, and it's adequate with my new front 1/2-ton calipers, which are larger than stock XJ fronts; if I could find an MC that had a little larger bore, that would fit my current stock single diaphragm booster, I'd swap again.
Any commercial disc brake-kit manufacture will tell you there is a difference in an MC intended for drum rears versus one designed for disc rears, and it's more than just the reservoir size.
Removing your prop valve insures it's not a problem. The only reason you may need one, is the rear brakes may lock up before the fronts. If that happens, then a prop valve is needed to reduce rear brake response.

Click to expand...

I never really looked at puting the mercury master on it before but if that is the way to go i will have to try it. I should also mention that the front axle is a dana 60 so do you think that mercury one will be adequate for that also?

I know the difference in drum to disc masters but from what I understand a e350 one would work but then again i could have misunderstood.
Thanks

...

DaJudge said:

Did you answer this question?

Click to expand...

Yeah I did in post 9. That is what it acts like tho.

I think the 1.125 bore will be marginal for 3/4-ton brakes in front and 1/2-tons in the rear. I say that because of my experience with what I have. As said before, I have 1/2-tons in front and metric Caddy's in the rear. I'm using the Mercury 1.125 MC Now, and I can lock up my brakes, but the pedal is too far down for me; too close to the gas pedal when my brakes are applied. Otherwise, my pedal feels fine, just moves too far to fully engage the brakes; this is a sign that the MC's volume is lacking. So, I'd think with your brakes, the pedal would drop even farther. I don't know what MC would have a 1.250 bore and fit the early XJ booster. But, even if you found one, I'm not sure the pedal wouldn't be too hard, requireing the move up to a dual diaphragm booster to regain the pressure lost by going to the larger MC. For me, I'm planning on swapping in a late XJ dual diaphragm booster along with a '99-03 Dodge Ram 1500 MC--which has a 1.250 bore. I've read this works, but there may be clearance issues with the XJ hood. At least one post stated it worked with an MC manufactured by Bendix. Seems the differences are in the reservoir height. Basically, I think you have air in the system, still. And I also think, even when properly bled, you'll find the pedal soft, and low.

Just looked up '76 Ford F350 (Dana 60??) front discs; they have 2-pistons, 2.18 OD inches each, vs '76 Chevy front 1/2-ton calipers, which are single piston, 2.94 inch bore. If your D60 has the dual piston calipers, they are quite a bit larger, in volume, than the 1/2-ton units; however, since you proved your current issue is with the rear, by capping the rear circuit at the MC, resolving the issue in the rear is first order of business. I suspect, you get the air out, and you'll have brakes.

Thanks for all the info. I had my buddy order up the mercury one. I'm heading out to work on it right now and see if I can get it bled better otherwise the merc one will be here in the morning and we'll go that route.
Thanks again!!!