Disc Brake Conversion Failing

-
Checked and rechecked for leaks, found nothing. I think it's the metering valve.

It is physically impossible for the metering valve to lock one rear brake and not the other. All it does is change the pressure that goes back in the rear brake line. A single line. That feeds BOTH brakes. Think about it.
 
Yes, that is true. But, like I said, that's BOTH rear brakes. If only one is locking, that problem is between the T block on the rear axle and the rear brakes. Even if the metering valve is messed up, it would effect both rear brakes equally. It sounds like you have more than one issue here.



The metering valve requires the brakes to be bled in order, so that the metering valve sets correctly. Passenger rear, drivers rear, passenger front, drivers front is the correct bleeding order.

But even if the metering valve were misadjusted, it would effect both brakes equally. In the rear at least you have one problem that's between the "T" and the rear brake. Air, brake line issue, brake line adjustment or contamination.
at best or worst it might be air. what do you mean by brake line adjustment? how do you adjust the line and what effect would that have on the brakes?
 
It is physically impossible for the metering valve to lock one rear brake and not the other. All it does is change the pressure that goes back in the rear brake line. A single line. That feeds BOTH brakes. Think about it.
yes but wouldn't the metering valve also take pressure away from the front? I agree with you that there's a problem in the rear, but I don't think that one problem would impact the front as well. again I'm here for advice, so any and all advice is appreciated.
 
yes but wouldn't the metering valve also take pressure away from the front? I agree with you that there's a problem in the rear, but I don't think that one problem would impact the front as well. again I'm here for advice, so any and all advice is appreciated.

No. The pressure to the front isn't changed. There's a sliding piston inside the metering block that moves when you bleed the brakes to adjust the pressure to the rear line.

You have to be systematic about this and fix one thing at a time. You know you have a problem at the rear brakes, you have to fix that and then move on to the next one.

I don't know what it is that you did to the rear brakes, but if they both worked before, and only one works now, there's really only a few options. If you didn't adjust the rear brakes, or change any rear brake components, then you probably have something in the line- air, rust, chunk of whatever, etc that's effecting one side. If you did alter or change stuff out on the rear brakes you'll have to get deeper into it.
 
Pull the rear drums and inspect of leaks and foreign material
on the brake shoes.

Some of the Masters look similar BUT have different front to rear
proportions built into them
IE: 61 to 39 or 69 to 31 are two I remember.

I use 61 to 39 that was a Mopar Perf part.

Also make sure residual pressure valve is in correct place on cylinder

That being said my son does all our brakes as I am no expert on them!
 
The car is 67, I did a 68 disc brake conversion up front. the brakes were stopping but not locking and really doesn't feel much different from drum/drum application and the pedal does feel pressure

  1. How did the car stop before the conversion?
  2. Are the front calipers Kelsey Hayes, 4 piston?
  3. We're the calipers rebuilt?
  4. How long have you been driving a manual brake car?
  5. Did you do the propper brake in of the pads / shoes?


I replaced the drum/drum MC with a disc/drum MC

I think we have established that the MC is a DRUM / DRUM. The drum / drum MC has a residual check valve in both ports. If there is one in the port you have attached to the front discs you might be dragging the front brakes heating them up causing them to glaze or just plain overheat.



also replaced the stock distribution valve for what I think would be the correct proprtioning valve

If you got a complete 68 system you will have the distribution / safety switch and a proportioning valve. No metering valve.

In the 67 thru 70 something KH system, the distribution block for disc or drum is the same. Down stream in the single line to the rear brakes is a separate proportioning valve. It takes the line pressure from (just numbers for example) 1000 psi and reduces it to 300 psi. That way the braking force for the front is in line with the rear. Otherwise the rear would lock up way before the fronts and you would be switching ends when you try to stop.


Welp after all of this only one wheel is locking and it happens to a rear one at that

The adjustment of the rear shoes could explain this. Or shoes not broken in propperly, the backing plates could have worn spots where the shoes ride causing them to stick. Air in the system, Lots of reasons.


Pleeeeaaase help. MC was bench bleed, no air in the system (I've bleed and bleed the system) new shoes in the rear, new pads up front, new everything and no locking

Locking up is not the benchmark you should be looking at.

When the wheels lock they are sliding and it takes longer to stop, plus you loose control.

You want the brakes to not lockup but stop the car in the shortest distance.

There is a lot going on here and a lot of good advice. as someone already said the braking system is a system. Take it one step at a time.

I know they are corney but watch this video




Good luck.
 
Last edited:
Would a car without a proportioning valve run a larger resivor to increase line pressure to the front? Totally Spitballin' here. Just wondering.... but yes, these people MAKE mistakes, and even sell crap that dont work. Knowingly....

Reservoir size has no bearing on line pressure. The reservoir is larger because the pistons in a disc brake system are larger than wheel cylinders and as the pads wear, the level of brake brake fluid can get too low if the reservoir is too small.

Only the bore diameter affects line pressure.

You ALWAYS want FRONT brakes that can lock on clean dry pavement, that's how you know that they're achieving maximum stopping power. They need to do so predictably. Then riding the edge of lockup (which is the point of minimum stopping distance) is up to the driver or, in later systems, the abs system.

You NEVER want the rears to lock before the fronts That results in a spinout.
 
Last edited:
Surprised nobody has mentioned about the Dodge Darts with the 8 3/4 rear ends that have a history of the factory size 10" rear brakes coming on early with the disc brakes up front. (rear brakes come on early and can spin the car in a circle under heavy braking).

The fix was to put in smaller size rear wheel cylinders.

Here are 2 choices:

Doorman 7/8" bore
rear wheel cylinders.
20220902_234741.jpg


Raybestos 13/16" rear wheel cylinders
20220902_234629.jpg

20220902_234557.jpg


Also here is a 15/16" bore Disc/Drum master cylinder to fit the A Bodies.
Mopar used a 1-1/16" bore for manual discs in A-bodies, but that makes for a much stiffer pedal than most people are comfortable with. 15/16 bore reduces the pedal effort to a more reasonable level.


Out of stock at Rockauto right now.
20220903_000245.jpg

20220903_000312.jpg


MC36406 is available on Amazon
15/16" Bore Disc/Drum manual brakes.
15/16" bore, pedal easy to push.
20220903_001648.jpg


(or call Dr Diff as he has options too)
 
Last edited:
Surprised nobody has mentioned about the Dodge Darts with the 8 3/4 rear ends that have a history of the factory size 10" rear brakes coming on early with the disc brakes up front. (rear brakes come on early and can spin the car in a circle under heavy braking).

The fix was to put in smaller size rear wheel cylinders.

Here are 2 choices:

Doorman 7/8" bore
rear wheel cylinders.
View attachment 1715979980

Raybestos 13/16" rear wheel cylinders
View attachment 1715979981
View attachment 1715979982

Also here is a 15/16" bore Disc/Drum master cylinder to fit the A Bodies.
Mopar used a 1-1/16" bore for manual discs in A-bodies, but that makes for a much stiffer pedal than most people are comfortable with. 15/16 bore reduces the pedal effort to a more reasonable level.


Out of stock at Rockauto right now.
View attachment 1715979983
View attachment 1715979984

MC36406 is available on Amazon
15/16" Bore Disc/Drum manual brakes.
15/16" bore, pedal easy to push.
View attachment 1715979985

(or call Dr Diff as he has options too)

Nobody's mentioned it because he's only locking up one rear brake. He's got bigger problems than the minor factory balance issue.
 
Right rear wheel locks up first under hard braking cause there is less weight on it, no one in the passenger seat to "Balance" out the load.
 
Right rear wheel locks up first under hard braking cause there is less weight on it, no one in the passenger seat to "Balance" out the load.

Only if set your ride height and alignment with no weight in the drivers seat.

And this is a new problem for the OP after a disk brake conversion, not a general characteristic of the braking system.
 
The proportioning valve on these cars is a separate small unit sitting half way between the front and the back wheels.
 
[1] Disc brakes require about 50% more line pressure than drum brakes. That is where the problems start.
[2] The factory used a number of methods to make the discs & drums work as a happy couple....Proportioning valve to limit pressure to rear brakes; changed the bore size of the rear wheel cyls; or both.
[3] One rear wheel locking. The 'locker' could actually be working correctly in this mismatched braking system; the non-locker could have grease on the linings, leaking/rusted up wheel cyl. Etc.
[4] The rear wheel cyl bore size needs to checked to see if it is correct for the PV & the car. Then the rear brakes need to be closely examined for mechanical problems that could cause lock up & considering the small &&&&&, new wheel cyls fitted.
[5] M/C bore needs to checked to see if it is correct for that disc/drum combo.
[6] One reservoir of the m/cyl is often larger than the other. For an enthusiasts car where oils/fluids get checked, it doesn't matter which chamber you use. Arrange according to your brake lines if you want too. It makes NO difference to the braking capability. Res line pressure valve was ONLY used with drum brakes, later deleted as unnecessary on later cars. Likewise the useless metering valve, just something else to fail. GM deleted these, not sure about Mopar.
 
I don't care what peoples opinions of it are, I don't like that and I don't think it's right. I agree the pressure won't be affected, but it's still not right.

It's a yuge safety issue. Yuge I tell you.
 
I don't see what the big deal is...or safety issue is. The two are obviously different [ from different manufacturers? ] & the shape is different. It is how they operate that is important. While the reservoir divider position is slightly different, the fluid volume that each section contains could be the same or very close because the internal shapes are different.
 
I don't see what the big deal is...or safety issue is. The two are obviously different [ from different manufacturers? ] & the shape is different. It is how they operate that is important. While the reservoir divider position is slightly different, the fluid volume that each section contains could be the same or very close because the internal shapes are different.
The big deal is that they sold me a MC that is supposed to be for a manual disc/drum application and it is actually a manual drum/drum application. I already had a perfectly working drum/drum manual MC and didn't need another one. The car is now set up for manual disc/drum so I need an MC to fit those needs.
 
Yeah I seriously doubt the reservoir size is what's causing the issue here. Unless the intake holes are sucking air because the fluid level is too low with a full brake application the reservoir size is not super critical. The line pressure is determined by the bore diameter of the master cylinder, and ANY of the mopar MC's should effectively activate the brakes because it's only a swing from 15/16" to 1-1/32". Pedal feel will change in that range, but all will stop the car. And the residual pressure valves shouldn't be an issue. More importantly, it's really unlikely to be the issue unless the MC is actually defective (which it could be…). If it didn't have a rear residual valve it would be difficult to get either of the rear drums to lock up. If it had a drum residual on the disk side then the front disks would drag, which isn't the current issue.
 
I did the drum to disc swap last summer. I haven’t gotten around to the proportioning valve. I bought an adjustable one but I’m hesitant to cut my line and make another potential place for a leak.
Here’s my question… Is a factory proportioning valve a direct replacement for the drum drum distribution block ?

A8D48D62-178F-443D-8BAA-47812CD46FED.jpeg
 
I did the drum to disc swap last summer. I haven’t gotten around to the proportioning valve. I bought an adjustable one but I’m hesitant to cut my line and make another potential place for a leak.
Here’s my question… Is a factory proportioning valve a direct replacement for the drum drum distribution block ?

View attachment 1715980703

Not that I know of, and you may not need to install the adjustable valve. With the extra meat under the back of your car that I can see in your avatar, I'd go do some aggressive stops on an EMPTY, straight, flat, rain slicked road, and see if you can even get the rears to lock. If not, you don't need it.
 
-
Back
Top