Master Cylinder upgrade (post 97 XJ's)

Is there a larger master cylinder out there that could be installed with minimal modifications to '97 and newer XJ's in order to improve braking performance?

I read that some YJ's can improve braking using older (70's) full size Ford cars (see link) http://www.4x4wire.com/jeep/tech/brakes/yjmc/

I wonder if a similar conversion exist for those of us.
As long as the volume of the new MC is larger than OEM you would think performace would increase.

I have a corvette master cylinder (early 70's). I had to fab a push rod from 1/4" x20 all thread and used a nut to fix the depth in the booster.

97
SeanP

Seanp--are you saying you installed an early '70's Vette MC on a late model XJ? And the only thing you had to do was extend the push rod?

yes. I also had to bend and flare the lines that go from the MC to the distribution block. Not a problem once I got practice at doing the double SAE flare.

SeanP

How much bigger is the Corvette's MC versus the XJ oem?
Did it reduced the required force on the pedal?

it;s not about pedal force, it's about moving a greater amount of volume for the bigger 1 ton (ford twin piston) and chevy 3/4 ton (sinlge piston) calipers. IIRC, the vette MC is 1.125"

My understanding is if you get a MC with a larger bore when it is used with stock calipers/drums you will end up with a very hard pedal, but not necessarily better brakes.

which poses the question: Why would you want to upgrade the stock XJ brake MC? Expecially the 97+, stock XJ's had pretty good brakes and were well balanced from the factory. The only reason to swap in a bigger MC is to push more fluid volume into larger calipers.

Seanp upgraded his front/rear calipers to larger pistons than stock, so he needed more volume to stay in balance. A larger size piston MC swapped into an otherwise properly balanced system would result in a hard pedal.
I've been searching for a larger MC, such as the Vette, or Mercury Grand Marquis (both 1.120 pistons), that would adapt to the dual diaphragm booster found in late model XJ's, and Grand Cherokees. I was led to beleive the early MC's, like the Vette, would not properly seal on a dual diaphragm booster. Seems Seanp has proved otherwise.

i had to get some gasket material and cut a custom gasket and RTV both sides. no worries there.

There is another school of thought that an E350 master cylinder would work without changing the push rod depth, but you are going to want to get that adjust so that there is about 1/8" travel before pushing the piston. You definately DONT want the pushrod too long and engaging the piston without pressing the brake pedal. Upgrading the brakes is a good thing with larger roating mass of 35"+ tires.

The reason for the original question was to improve breaking performance in my '98 with ABS and front Powerslot disks.

I feel that in order to come to a normal stop, harder than normal force must be applied to the pedal. Pedal travel seems to be excesive as well.

I am only running 31's and I recently installed a new booster and bled the system with little to no improvement felt.

That's why I was focusing on the MC next.

But from the repplies it sounds like an upgrade to the MC alone wont help; it would require larger caliper pistons and disks which would require larger knuckles as well.

BTW: I've been driving it for 3 years now and it always been this way. The powerslots helped a bit but still not to the standard I was hoping to.

Didn't think '98's had braking problems in stock trim. I wouldn't think 31's would significantly affect braking on your '98. Poor XJ braking is generally associated with the pre-'95 XJ's, which came with a single diaphragm booster. I beleive in '95, a dual diaphragm booster was used, along with improved front calipers; although, I'm not sure about the addition of improved calipers. By itself, an MC with a larger than stock bore will cause a harder and a higher pedal. Meaning a lot more foot pressure required to apply the same force to the brakes as you could before the MC change. You first need to find out if '98's, as a class, have poor brakes even when new. If yes, then larger rotors with larger calipers will help, along with a larger MC to satisfy the new volume requirement. If no, then you need to look closer at your current system to identify the problem. You may not need to spend the money/time upgrading, unless you've added 33's and a lot of additional weight (bumpers, skid plates, winch, etc). Another upgrade would be a larger diameter dual diaphragm booster--that is if you have room on the firewall to mount it. I've also read on this and Pirat4x4 forums, that the Grand Cherokee front knuckles & discs will swap over to an XJ's front D30 axle. That would be an improvement worth considering.

I am starting to think that I may have a different problem in the break system, unrelated to the MC/booster assy.

I think your right. Have you adjusted your rear brakes lately, and re-bled the hydrolics?

check to see if you have a kinked line somewhere as well. that would cause a huge restriction even if it is small.