Master Cyl Swap from 90 Cherokee

[pflaming]

Update: my new MC and lines now two years old with a fire storm included. Took it for a test run two weeks ago brakes are still solid and secure. Drives aas it did pre-fire but a bit windy without a "wind screen"! Motorcycle goggles less expensive than new glass and easier to clean.

[vance1234]

Thanks 4mula-dlx.  Do you think your master cylinder would work with the original drum brakes?  That is just too funny pflaming.

[NiftyFifty]

The master will work fine, but the proportioning valve would most likely not be right for front drums, but you can pick up an adjustable one for fairly cheap from Summit or Jegs etc.

[Fargone]

I know some guys asked about a master swap that was of size and not a custom job and I managed to find one off of a 90 Cherokee at the local wrecker that was the same length as original and proceeded to see if it would work.

Without much effort at all it fit nicely into place. So here's the procedure:

Remove the original master and then remove the push rod and yoke right from the pedal arm. This will need to be adjusted or cut later, as I can't speak for every model and I don't know what rod or length you have, but my old push rod slides right into the new master perfectly and basically the bore isn't changed as the size is the same.

Get a piece of 1/4" flat steel plate and trace the mounting patern of the original master cyl. Cut the edges of the plate to make it close to the original size of the backing plate on the master and mark and drill the three original spaced holes traced from the orig. master cyl. I think their about a 3/8" hole, but you may want to go just one step bigger for a bit of wiggle room to get the bolts in.

Then quickly placing 3 bolts through the holes into the old master, carefuly trace the outside of the bore of the old master and cut that hole "approx 3" with a cutting torch or whatever tool you have. Now place the new master onto the plate with the 3rd hole to the bottom and mark where the 2 new mounting points need to be: Now drill out the holes to 5/16" and use two regular or carriage bolts pushed through from the backside of the mounting plate. I welded the bolts on then ground everything fairly smooth, and welded and grinded until I had little showing above the plate, but enough that there was still strength of the original bolt head and the weld.

Install the plate to the truck with only the 2 top holes receiving lock washers, or use longer bolts and use lock nuts on the back of them to secure. For the cherokee master cyl I had to grind just a tiny bit off the bottom bolt head edge to get it to fit tight to the plate but it's very minor and won't affect the strength. The only other issue is one of the front line to the proportioning valve is in the way of the steering clamp and just need a new line made and re routed from the bottom up rather then around the end. (pics below)

Now feed it through the top with the mounting side facing down and slip onto the new mounting studs and put on locking nuts or lock washers and tighten. This does require some changes from the original line, but you can purchase felxible line and adapters at most jobber spots, for me I ran all new line so wasn't an issue.

Hope this helps anyone looking to go to a 2 res system.

Wow now that's the kind of detailed reference material I like. Thank you so much it looks like I'll be making those changes as well. I've even got a line on a Grand Cherokee diff complete with disc brakes.

[55 Fargo]

Moved this post to the thread discussing Vacuum boosters.

Edited April 25, 2016 by Fargos-Go-Far

[NiftyFifty]

Let's not muddy this thread with booster discussion....it's a mute point unless your willing to relocate your brake pedal and then has zero to do with this swap, as a Cherokee booster is much too large.

I'm glad I could help Fargone, this was a great addition to my build and gives the feeling of newer brakes...just be mindful if you get a diff with rear disc, to look for a master set up from the same vehicle so the proportioning valve is the same

[Jeff Balazs]

Let's not muddy this thread with booster discussion....it's a mute point unless your willing to relocate your brake pedal and then has zero to do with this swap, as a Cherokee booster is much too large.

I'm glad I could help Fargone, this was a great addition to my build and gives the feeling of newer brakes...just be mindful if you get a diff with rear disc, to look for a master set up from the same vehicle so the proportioning valve is the same

Actually if you try to use just the Grand Cherokee M/C with the Grand Cherokee disc brake rear axle you will end up with a rear brake bias. You need to add in an adjustable proportioning valve so that you can balance the system. Without this added component it will stop quickly but feel like it wants to swap ends on you.

 

Jeff

[pflaming]

I put a chrokee rear axle under my truck which has drum brakes, works fine but for the same amount of work I could have had discs. I drove it with the rear brakes only, only two blocks, the rear brakes at best provide about 30% of the braking, just enough to keep the rear end behind the front end. Think of the vehicle as an arrow, which end has the MOST weight, the leading end and that's that mass that must be stopped.  

[NiftyFifty]

I used the stock proportioning valve on my rear disc set up and it works great...no front or rear grab, very nice even braking....maybe disc works different, but in reality it really shouldn't if you use the rusty front and the jeep rear.

[Jeff Balazs]

I used the stock proportioning valve on my rear disc set up and it works great...no front or rear grab, very nice even braking....maybe disc works different, but in reality it really shouldn't if you use the rusty front and the jeep rear.

Well I thought so too. Until we did a bunch of testing. I think it is real easy to get fooled by disc brakes ....especially on an archaic old truck. They will feel nicer than the stock brakes. Fortunately a buddy of mine had convinced me to put a Wilwood adjustable valve in so that I could fine tune the reaction from front to rear. He and I spent 3 or 4 hours testing it on dry and wet pavement one weekend. Initially I thought it felt very good with no tendency to lock up at all. And this was before we made any adjustments. In other words with the valve wide open. In reality the rears (which are larger than the fronts) were picking more than the fronts. It was almost impossible to tell this from inside the cab.......but watching it from the sidelines you could see it clearly. Especially on wet pavement. And when my buddy did a couple of extreme panic stops on wet pavement in front of me I knew he was right all along.

 

If you think about it this it really isn't so strange. Even with a M/C that is the exact one that was matched up in production with the calipers on the rear axle. The front calipers are from a earlier GM car.....and the whole package probably has slightly different hydraulic volumes and certainly different weight distribution than either of the donor vehicles. It makes sense to build it so you are able to adjust for this. You don't want to find out the hard way on a real panic stop that you have done all this work but it really isn't right.

 

Jeff

[NiftyFifty]

I've had a few hard stops for deer and dumba$$es, and so far been really happy...but again I used a jeep rear drum axle and a mc off a drum Cherokee and it seems to do well....I can tell my front brakes lock first if I hammer on loose gravel on the pavement. Nothing wrong with an adjustable valve, you can definitely fine tune..but after 6 years with mine, I'm pretty comfortable with it. I wish my 67 Monaco brakes felt s good...I always feel like I'm on the floor with it, but Chrysler just had crappy feeling brakes in that era

[NiftyFifty]

Just as a note, if you do do a MC swap on a rear drum set-up and use an adjustable valve you MUST use a residual valve in the feed line to be back...I think it's a 5lb you need? If you don't the cups on the wheel cylinders will leak..they need constant pressure to keep from leaking, on a rear disc this doesn't matter...and in the stock valve I used, it's built into the valve.

[Jeff Balazs]

If you throw rear discs into the equation then I feel very strongly that you need to have the ability to make this adjustment.

Now that I have a few thousand miles on this I think it is time to readjust the valve. It is starting to feel like the rears are picking first.

Those calipers appear to be a more efficient design than the fronts. Again not surprising as they are from a different technological era than the front calipers.

I originally put residual valves in but they were not needed with the discs all around. They had to be removed as they cause significant drag.

 

Jeff

[TylerB46]

On 7/18/2012 at 12:29 PM, NiftyFifty said:

i always get pm's asking how the set up is working, and I can report that I'm in the middle of my 3rd year and the cherokee master is still working great and the brakes feel good with no noticeable grab on one side and a Deffinate feel that the front are doing their job as the main stopping force.

I did this whole set up with the engine trans in, but if out you could possibly skip the plate mount and drill holes in the actual cross member

If you do skip the plate mount and drill through the cross member, would you still have to lengthen the plunger?

[pflaming]

It's been sometime since I put mine in. If you drill holes in the crossmember you will have to tap them for the MC stub bolts. I wonder if that crossmember would be thick enough to hold? 

[TylerB46]

46 minutes ago, pflaming said:

It's been sometime since I put mine in. If you drill holes in the crossmember you will have to tap them for the MC stub bolts. I wonder if that crossmember would be thick enough to hold? 

Instead of tapping them, could I just run long enough bolts to go all the way through and then put bolts on them?

Edited December 2, 2018 by TylerB46

[pflaming]

You will have to study the cross member. I don't know. I believe the MC must be perfectly square to the push ROD so that it pushes straight. The spacer mount assures that. 

[TylerB46]

12 minutes ago, pflaming said:

You will have to study the cross member. I don't know. I believe the MC must be perfectly square to the push ROD so that it pushes straight. The spacer mount assures that. 

I’ll look at it

[NiftyFifty]

It’s been 8 years now, so hard to remember what it all looks like, but the original just uses the thickness of the mount, so no reason the new one couldn’t, but as I recall it would be near impossible to drill it out in the truck properly.