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As part of my rear disc brake conversion, I bought an external 2psi residual valve from Wilwood.  My plan was to remove the stock drum residual valve from the master cylinder and use the inline valve, but I just realized something:

1. My aftermarket disc/drum master cylinder has left and right outlets, two for the front line and two for the back.  I guess the idea was to give options for left or right brake line outlets if space was tight.  I have one port on each half of the MC plugged.

If I'm not mistaken, that means that I have two drum residual valves (10psi) and two disc residual valves (2psi) already in the MC.

2. Could I just pull the unused 2psi valve out of the plugged port for the front MC pot and swap it into the back port where I now need another 2 psi valve? It seems like this would accomplish the same as the external valve and would save me from making two more flares that will probably leak.

Any thoughts are appreciated.

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Case...As long as you're sure it's a 2 PSI valve, and if it fits properly in the master cylinder line port, this might work.  You can test residual pressure, either with a gauge or actual "feel" at the rotor.  2 PSI is distinctly different than 10 PSI.  Before making these changes and using a drum brake port for the new disc rear brakes, read my comments below about fluid displacement volume and pressure demands.

Could the reason for the four ports on the aftermarket master cylinder be a high performance feature designed for steering brakes that use manual controls and valves for each wheel?  Otherwise, this would seem like overkill.  One port for each end of the vehicle, if the line size and fluid displacement volume are sufficient, would be more conventional.  Another possibility is fluid displacement:  One port for each wheel might be set up for additional pressure per wheel.  One more possibility, depending upon the fluid routing and reservoir supply for each port, could be a safety feature:  Each wheel has its own fluid supply in the event of a line failure at a single wheel.

In any instance, know whether the master cylinder requires splitting fluid to each of the four wheels.  You need enough master cylinder fluid displacement per wheel to safely move the disc brake piston and apply the brakes!  Disc brakes can require more fluid displacement and different pressures than drum brakes...Be safe here.  Determine whether the disc brake ports displace more fluid volume than the drum brake ports.  Check the reservoir capacity with regard to disc brake demands.

Please share more about the actual design and purpose for this master cylinder...

Moses

Edited by Moses Ludel

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Moses,

Lots of changes since the last post.  The master cylinder I was using came with the dual diaphragm booster kit I purchased last year.  It is pictured below.

This is sold as a Jeep CJ kit.  They had disc/drum or disc/disc options.  Obviously, the latter was never a factory option, but I don't think a dual diaphragm booster was either.  I went with disc/drum at the time, hence I assume I had 2PSI and 10PSI residual valves in the MC.

I did look to see if I could switch around the valve from the spare port as I previously mentioned, but the front is a 9/16" outlet and the rear is a 1/2" outlet, so I wasn't going to be able to use the disc residual valve for the the rear MC pot going to my new disc conversion.

I already had a Wilwood external 2PSI valve, so no big deal.  I was just trying to save a few bucks, a few extra flares, and have what I see as a more elegant solution, i.e. the correct valve IN the MC itself.

I attempted to follow the standard convention of screwing a machine screw into the brass seat to remove the 10PSI residual valve from the MC outlet.  I think I've even read this technique in your book somewhere.  Let's just say I did not have spectacular luck.  45 minutes and one slide hammer later I never did get that brass seat removed.  I didn't have an E-Z Out small enough, so I wasn't able to check to see if that brass seat was perhaps threaded.  Being it's such soft metal, I just ended up destroying the brass seat and never did get the valve out.

I have read numerous other threads that recommend Corvette master cylinders as good options when upgrading Jeep braking systems.  The trouble is, there are about as many opinions on that subject as there are Jeeps.  I did get some info posted by the owner of Vanco Braking Systems.  For a different user several years ago, he recommended a 1977-1982 Corvette MC for use with front/rear disc setups on Jeeps. These were all 4 wheel disc cars, so the residual valve should be correct.

I was able to source one locally. It looks nearly identical to what I got on the MBM kit pictured here, except it has only two ports vice 4.  It is a 1 1/8" bore versus a 1" bore on the older MBM kit I already installed.

Bench bleeding was a bear, but I think I did OK.  The brake feel isn't as good as it was; it requires a bit more effort.  I assume this has to do with the change in bore on the MC.  The pedal is firm, and travel seems reasonable, so I'm OK with it for now.

By the way, the rear calipers are from a Nissan 300ZX application from the late '80s.  They use a relatively small piston and have an integral mechanical actuator to squeeze the pads to the rotor for parking brake.  One of the reasons people use this caliper is to not outpace the amount of fluid the MC can supply.

I did install an adjustable prop valve to dial back the rear brakes if needed.  I don't think I'm going to be able to lock up the brakes on dry pavement, so I'll have to wait for some rain or find a dirt road to see if I have the adjustment OK.  No leaks yet, so I'm reasonable happy. 

It's not a tremendous increase in braking performance, but I got it all for about $150 plus the $42 master cylinder I damaged, so I figure it's not too bad for a disc swap. Also, the parking brakes work better than the old drum one ever did, so that's a plus.  Time will tell how it all works out.  I may bleed again in a week or two to see if that changes the brake feel at all.

 

Dual diaphragm booster.jpg

Rear Disc.jpg

Edited by 60Bubba

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60Bubba...Pleased to know that the piston size at the rear disc calipers will not require too much fluid volume from the master cylinder.  You're right about the 1-1/8" piston increasing the apply pressure slightly, though the booster should offset that change.  Make sure you have correct clearance between the booster's apply rod tip and the master cylinder's primary piston recess.  You want the master cylinder pistons to retract past their reservoir compensating port(s) when the brake pedal is released.

It's reassuring to have a fully mechanical parking brake.  The caveat still applies: Avoid locking up the rear brakes if the front brake system fails.  A classic stunt car maneuver is to lock up the rear brakes (only) and force an immediate spin-out of the vehicle.  This stunt does not work for higher center-of-gravity vehicles like a Jeep CJ, as the risk of a rollover increases dramatically!  These stunts, in any case, are generally performed on a wet skid pad.

That brass port tube seat in the master cylinder should come out; rebuild kits "in the day" included new tube seats for these master cylinders.  These port seats are often tapered for a press fit that gets reinforced by the flare nut/tube tightening torque.  When factory pressed into place or installed snugly with the tube and flare nut, the seat can really take a set.  This may well have occurred on your cylinder.  A screw extractor, cinched into the brass seat's fluid passageway, can sometimes be tapped sideways to rock a tapered tube seat loose.  (The rebuild kits often included a metal screw that fit the tube seat passageway.  The screw could be pried outward, using two opposing screwdrivers beneath the screw's head.  As a last resort, your technique with the slide hammer might work.)  

Caution: Always vacuum and/or flush any brass debris from the port to prevent the debris from entering the hydraulic braking system.

A quick search online turned up a source for master cylinder brass replacement "tube seats".  Scroll down the FAQ for a photo of the seats, you'll see how a tapered "peg" wedges the seat snugly into the iron cylinder:  http://www.brakecylinder.com/BrakeFAQ.htm.  These tube seats set firmly in the cylinder—as you discovered!

Moses

 

Edited by Moses Ludel

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Much appreciated info!.  I didn't have to turn the MC in for a core, so I still have it.  It was less than a year old, so the brass seat is the only problem.  I'll probably fiddle with it some more to see if I can get it out.  If so, I'll replace it and have a spare.  Additionally, if I really don't like the feel of the new one, I could always put the 1" MC back in for a direct comparison.

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60Bubba...Glad to comment...Brake safety for your family is paramount, you do great work and will continue to do so, Case!  Will be good to have a backup master cylinder and choice of piston bore size.  

These double bail iron master cylinders are a classic design, G.M. at its finest hour.  Our '70s and '80s G.M. trucks still bring a smile in the family album.  Favorites were the 1973 K10 4x4 SWB with SM465 4-speed transmission and NP205 transfer case plus the 1986 (carbureted 350) and 1987 (TBI 350) Suburban K2500 3/4-ton 4x4s with THM400s.  You've got a piece of that golden era in front of your brake booster.  

Thanksgiving for your family this week!  Must be real fall colors in your neighborhood...

Moses

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After reading the comments here it sounds as though the project I've set myself up with may take a little more research.  Moses, I purchased your book Jeep Rebuilders Manual and discovered your discussion on front disc brake conversions.  Without further investigation (except to watch online videos which made sense and looked like a great weekend job) I ordered a complete front disc brake system without MC.  Your discussion on page 392 of the topic sounded like I would be able to keep my current MC in place and just remove the check valve and spring for the front brake line source.  I'm I correct or I am going to have to find a disc/drum MC?  As you can tell this is my first time doing a brake conversion.  My vehicle is a 1973 CJ5  

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Hi, tpoulson, and welcome to the forums!  For use with a disc brake conversion, the check valve on a drum brake master cylinder would need to be removed from the port going to the front disc brakes.  (Rear brakes with drums and shoes still require a check valve and residual pressure.)  Disc brakes require either very slight or no residual pressure, otherwise the caliper will apply pad pressure and drag when the brakes are released.  

Note: Some later disc brake systems actually use a very slight amount (much less than with drum brakes) of residual pressure just to keep the pads aligned with the rotors—this is very low pressure, not enough to create brake drag or pad wear.  (See fellow member 60Bubba's comments and my replies at the beginning of this topic.)  Earlier disc/drum systems generally eliminated the check valve at the disc brake port of the master cylinder.  Master cylinder parts schematics and factory shop manuals clarify the OEM design.

As for whether you can use the 1973 drum brake master cylinder and simply remove the check valve to the front disc brakes, an additional concern would be displaced fluid volume.  You need to be sure that the master cylinder bore and fluid displacement are enough to satisfy the front calipers' demand.  That would be determined by the type of calipers you are running (number of pistons and their diameter), the master cylinder's bore size, and the master cylinder's piston stroke length/movement.  In addition to math, a rough test is to note the pedal travel needed to activate the front calipers and apply sufficient pad pressure at the rotors.

What type of rotor and caliper are you using here?  1977-78 is the beginning of the CJ disc front brake era.  Compare your '73 master cylinder's bore size to the '78-up master cylinders.  (You can determine bore size by the dimensions found in a master cylinder rebuild kit.)  Once your disc brake conversion kit is in place, test the system thoroughly in a safe environment, not in traffic.  See whether you need a front-to-rear proportioning valve; at the short 84" wheelbase, a proportioning valve may or may not be needed.  This often depends upon your vehicle's spring rate and shock absorber type.

Trust this helps...Enjoy the book!  Looking forward to your participation at the forums, we have an enthusiastic group of Jeep CJ owners.  We'd like to know more about your '73 CJ-5.  The 1972-75 Jeep/AMC models have unique needs that I'd be happy to discuss...

Moses

Edited by Moses Ludel

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Moses, thank you for your comments.  I'll use your guidelines and suggestions for my conversion.  Just one more question.  I read above how much trouble it was to remove the check valve and spring.  Is there a way to better way to handle the removal of the seat to get to the check valve and spring? It sounds like the seat is pressed in and will be ruined on removal.    

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tpoulson...In traditional master cylinder rebuild kits, new port seats were provided.  There are still sources of port seats for those who need to replace a worn or damaged seat.  I found sources with a simple Google Search.  Click on the link in my 11-22-2015 reply above...I can share more details if you need them.

Moses

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