Master Cylinder and Distribution Block?

Hiya -

I've got a dual circuit master cylinder and 4 wheel drums - I believe out of a 68 CJ5 in my flatty. I'm switching to disks in the front. Should I:

- Plumb in an adjustable proportioning valve for the rears
- Find a distribution block for a front disk car
- Leave it as is and cross fingers

Any experience is appreciated. When I did this conversion on a mustang I did the second option. Pretty easy fix and that's the way I'm leaning.

Thanks!

Bob

 

i say second option. it is fairly easy and well adjustability is never a bad thing.

 

Just went through the same process on mine. I kept the stock dual reser. MC as well as the factory distribution block. Note that it is not a proportioning valve but you already know that.

I then took apart the MC to try and remove the stock residual valves. I found that it did not have any (replacement MC from Napa). So I plumbed in a 10lb wilwood RV in the rear line and a 2lb wilwood RV in the front line.

Everything works great but I think I still have some air in the line because lately I've had to double pump the brakes

 

I like the adjustable ones.
But itd be easy and less $ to use a stock one.

 

This one is kinda hard. If you have a residual valve in the MC then it would be in the 10 lb range for drums. This may lead to the front brakes dragging BTDT. You will have to remove it and maybe run none, or if that results in a pedal with excessive travel then run a 2 lb or so inline residual for the front. I would go this far and then evaluate. You want the front to lock up slightly before the rear, with a pedal that is not spongy or has excessive travel. I can't see the rear locking up before the front after you have switched to front discs (ie front now better). If at some point you switched to rear discs than I would go with the adjustable prop valve in the rear, rather than hoping a stock prop valve will have just the right setting for your configuration, highly unlikely guessing you have some other mods affecting vehicle weight distribution, tire size, etc.

One of my experiences is converting to rear discs on a K5 Blazer with 3/4 ton axles. Of course it already had front discs, converted the rear and installed a adjustable prop valve. The prop valve was supposed to be able to reduce rear pressure up to 66%. I had to turn it almost all the way down (full 66%) to get the rears to stop locking up before the front, they were that much more effective. I did not change the residual valve and experienced no dragging.
On my cj with single circuit MC, I installed rear 11" and front disc. Front dragged so bad that it would hold the Jeep on a moderate incline!! Put in the clutch and it would drag to a stop. Never tried to see of I could pull the residual valve out of the MC and run inlines, that would have been a solution. Instead just switched to front 11" drums and have been very happy with the results. Of course I don't drive thru much water crossings, and don't do much mud running, the main reasons for converting to disc. The K5 Blazer was a mud racer.
My best advice is take it one step at a time to see what works, with a personal understanding that if it is even questionable you will take the time and money to get it right. This prevents "fixing it when it ain't broke" while keeping SAFETY FIRST.

 

Alright - so it sounds like a game plan is to pull the master cylinder and see if there are residual valves in there, if so - yank-o-rama. Then intall a 2lb in the front and 10lb in the rear. Cool. Thanks guys.

What would a residual valve look like inside a master cylinder?

 

Whoa, you only need to yank the residual valve for the disc brake side. Leave the one for the rear in. Here is the deal... residual valves maintain a slight pressure in the brake system to keep the brake shoes (drum) or pads (disc) against the drum or disc, so when you push the brakes, none of the motion is wasted moving the pad/shoe... you get instant brake action. Tracking? A drum setup needs around 10 pounds to overcome the pull of the return springs. A disc setup needs only about 2 pounds, as the only thing that pulls the piston back into the caliper is a slight drag against piston seal created as it moves out...once the brake pedal is released the seal wants to reutrn to its natural position, pulling back the piston slightly.

On a MC, the residual valves are located underneath the tube seats. In other words, remove the brake line at the master cylinder. See that surface where it seats? Pop that out with a skinny flat tip screwdriver or ice pick. The valve is beneath that, a rubber piece with a spring and check ball (I THINK). I have HEARD you can just jam a heated welding rod in there to disable that, but I wouldn't know. Be advised, this removing the residual valve stuff is only book knowledge for me....I have never done it!

This is why some will recommend replacing the MC with a disc/drum one... it is already setup from the factory with the right residuals....finding one to fit your application is the challenge...the right mounting configuration, the right size line hookups, right bore diameter (mainly determined by either power or manual application,but even then it can vary)

I have always wanted to experiment on a junk MC, just never have...

No residuals could result in the shoes being to far away from the drum/disc, resulting in a pedal that goes halfway down before anything happens, or maybe even needing to pump it once or twice...NOT SAFE!

Bottom line is it can be more complicated than it first appears...good thing is it can and has been done.

Hope this helps some, any more questions feel free, I have a little professional schooling about this subject and a little real world experience as well....
SAFETY FIRST!

 

The residual should be like shardein described. Instead of using an icepick to remove the tube seat I used a small self tapping sheetmetal screw. Just find one that is slightly larger than the tube seat opening and screw it in the hole. Then use a pair of channel locks to pull the seat out

 

Ok - thanks guys. I'll just pull the front one. Duh. I promise to get more sleep before making any brake system decisions.

Thanks for the excellent description. It'll probably be a while before everything is together and I get to experiment - but I'll letcha know how it turns out.

Bob

 

Ok - I still haven't got the tube seat out but I did manage to mangle it a bit.

I'm hoping the tube seat (which I assume I will have to replace) is something I can get at a place with lots of fittings (hardware store - good parts place). Yeah?

 

Ok - I figured out the problem. Did a little reading tonight and your description is perfect for a Bendix tandem master cylinder. I have a Wagner.

The tube seat on the Wagner appears to be part of a threaded plug that screws into the side of the MC. The residual valve was back behind that. So, I've unscrewed the plug/tube seat deal and pulled the residual valve. Now all I gotta do is figure out how to replace this tube seat I munged up and get a 2lb in line RV.

Hope this helps someone not make the same mistake.

 

...or maybe I can hit the JY and look for a car with front disks and a similar Wagner MC and snag the threaded tube seat insert and presumably ligher RV from behind it....