What if I don't change my Master Cylinder on a disc conversion?

[krazykuda]

Not really. Just Shows that some didn't do it right but just got lucky and never had s failure.

I hear ya Joe. That's what I've been trying to say all along. :banghead:


Some feel that they are right and will not back down, so I have stepped out to avoid arguements. I've said what I feel is the proper way to do it, but they keep insisting that I'm wrong. (funny how I can be wrong by telling them to do it right...) #-o

 

[longarm]

It appears there may be more than one way to make
the system work.
No one should take offence at a discussion that has
no true experts or engineers giving advice.
Unless of course you are an true expert or engineer,
then I'd ask you for your sources and research.

Maybe your opinions don't carry as much weight as
you'd like and your just sad.

 

[AJ/FormS]

The residual valve must come out of the front system.
-The job of the R-valve is to keep around 10 psi in the lines.When installed in drum systems it used to keep the wheel cylinder rubber cups slightly ballooned to keep them from leaking. Now if retained in the disc system, this pressure will tend to keep the pistons from retracting, and the pads will remain in contact with disc.The pistons are normally pulled back into the calipers by the square section O-rings by a process that is called seal-retraction. This force is quite small, compared to the residual valve pressure.This may be okay if they were working underwater, but not so good in atmosphere.The constant dragging could cause all sorts of problems.Just use your imagination.

 

[AJ/FormS]

And to CJP;
-I have the KH 4-piston/10x2.5 system,with the really small booster on my S-clone, street 360. I have 235s up front and 295s out back.DOT5 silicon fluid and a gutted P-valve.On the street,I have never wished for better brakes.They have even pulled me down hard from 93mph (car goes 93 in the 1/8th), several times in a row, without fade or drama. This system came off a 70 Duster with the Rallaye Suspension option.
-If you have a similar system, on an A-body,and for street,and its not stopping "as well as expected",I surmise that it may not be working correctly.
-If youd like to start another thread,perhaps we can help you work it out.

 

[equium]

The residual valve must come out of the front system.
-The job of the R-valve is to keep around 10 psi in the lines.When installed in drum systems it used to keep the wheel cylinder rubber cups slightly ballooned to keep them from leaking. Now if retained in the disc system, this pressure will tend to keep the pistons from retracting, and the pads will remain in contact with disc.The pistons are normally pulled back into the calipers by the square section O-rings by a process that is called seal-retraction. This force is quite small, compared to the residual valve pressure.This may be okay if they were working underwater, but not so good in atmosphere.The constant dragging could cause all sorts of problems.Just use your imagination.

That's good to know. So let me see if I got this right:

MC front port --->T fitting ---> R and L Calipers
MC Rear port ---->Prop valve ---> residual valve ---> rear axle brake drums

 

[moparmarks]

Back in 1987 I did a Chevy front disc brake conversion on my 1973 W200 truck. At that time I did not know that disc took a different master than drums. I left the drum master but installed a disc valve. For the next 15 years I 4-wheeled that truck bouncing it all over the place here in the Rockies. I never had any problems or loss of brakes. That being said I look back and feel very lucky. The manufactures had different masters for a reason. These days I always install a disc master when doing the conversion. Masters are cheap and why risk it?

 

[AJ/FormS]

And to those who depend on Chrysler to have " done it right", then why are we modding our cars at all?
-All our muscle cars would have had; way more power,more gears,bigger brakes,WAY more caster, stiffer Uni-bodies,factory exhaust headers,etc.
-Our cars would never have had 9 inch brakes,front drum brakes,7.25 diffs, open diffs,that lousy over-assisted P/S box,13inch tires,E70-14s on the back of any 340 car, etc.
-There was no "right"'rule book,back then. They were in the process of writing it. If they didnt have a rule-book, how could they know the "right way"?They did pretty good for the time. And they invented all kinds of stuff to put on different car lines that we can,today,pirate, to help make our cars,"right". Subjectively right that is.
-If they would have "done it right", I would have saved myself many thousands of dollars and hundreds of hours of work.
-Just my 2cents.

 

[moparmarks]

All manufactures of all makes used different masters with disc than drum. Not just Chrysler. Even cheap base line cars.

 

[AJ/FormS]

And to equium;
-Almost right.The residual valves are pressed into the exhaust ports of the M/C.I have never pulled one out. I have never converted a drum/drum M/C.
-IIRC that brass valve doubles as the sealing surface for the specially formed M/C brake line. That would mean, once you get it out, you would have to defeat it, then reinstall it so that the joint wont leak.
-Can anyone confirm that?
-I would NEVER tell you to just drill the R-valve out. You might wreck whats behind it. And you wouldnt know that till you drive it.And if a metal drill-chip were to get into the piston chamber, try to imagine what symptom that could create.Thankfully the M/C comes apart really easily.

 

[equium]

And to equium;
-Almost right.The residual valves are pressed into the exhaust ports of the M/C.I have never pulled one out. I have never converted a drum/drum M/C.
-IIRC that brass valve doubles as the sealing surface for the specially formed M/C brake line. That would mean, once you get it out, you would have to defeat it, then reinstall it so that the joint wont leak.
-Can anyone confirm that?

I misspoke with my earlier statement. I was confusing the residual valve for the Brake Warning switch. Let's try this again:

MC front port --->Brake Warning Switch ---> R and L Calipers
MC Rear port ---->aftermarket Prop valve ---> Brake Warning Switch ---> rear brakes

Since my car was originally Drum/Drum, I only have the Brake warning switch, not a combination/prop valve.

 

[Inertia]

And to equium;
-Almost right.The residual valves are pressed into the exhaust ports of the M/C.I have never pulled one out. I have never converted a drum/drum M/C.
-IIRC that brass valve doubles as the sealing surface for the specially formed M/C brake line. That would mean, once you get it out, you would have to defeat it, then reinstall it so that the joint wont leak.
-Can anyone confirm that?.

That's correct,..

Back when we used to overhaul these things,, it was standard proceedure to pull the seat out with a self tapping screw to replace the residual/joker valve (rubber),, along with all the other seals etc ...

 

[AJ/FormS]

Again,almost right.The P-valve should be after the brake warning lamp.For the warning lamp to stay off, it needs to have the same pressure on both sides of it.

-I just thought of something; If your car still has the 9inch rear drums, you might not need a P-valve. Those things have so little power.They might be a good match to the disc fronts.
-However, if youre running the 10x2.5 brakes,you will need the P-valve. Unless youre also running 10inch wide or better rear tires.

 

[equium]

Again,almost right.The P-valve should be after the brake warning lamp.For the warning lamp to stay off, it needs to have the same pressure on both sides of it.

-I just thought of something; If your car still has the 9inch rear drums, you might not need a P-valve. Those things have so little power.They might be a good match to the disc fronts.
-However, if youre running the 10x2.5 brakes,you will need the P-valve. Unless youre also running 10inch wide or better rear tires.

Got it. I'll plumb the Prop valve after the warning light and check my brake size too.
The PO included 2 sets of Duralast brake shoes, part numbers 330 and 331. One is a 9" and the other a 10". I'll know for sure when I remove the wheel.