Factory disc question for those with stock disc brakes. NOT aftermarkets

[63Tigerlily]

I just finished putting '76 Scamp front disks on my 63 dart and a 75 dart rearend with the 10X2.5" shoes. I told doctor diff my setup (essentially a factory disc brake car) and he recommended the proportioning valve I would need. I purchased a new dual master reservoir listed for a 76 scamp with disc front brakes. Everything is in, bench bled, and bled with no more air coming out. I am currently running manual, no booster.

The brakes do not toss me into the windshield as I expected. The pedal is a little spongy, with quite a bot of travel. They sem to stop OK, but the pedal is not hard. It pretty much feels like the 9" drum system I replaced them for. The only thing I did (due to a rounded off line nut) is that my proportining valve front circuit is connected through one front port to the factory distribution block, rather than the two front circuits being connected directly to the proportioning valve. I also understand that the master listed for disc brake fronts may very well NOT be that, as I purchased it from RockAuto.

For those of you driving factory manual disc A-bodies from the seventies, what do the brakes feel like? Has anyone experienced this before? I specifically chose factory components to avoid the all-to-common disappointment of aftermarket conversions.

 

[junkyardhero]

a lot of this hinges on the master cylinder bore size. do you know what you wound up with?

i don't know exactly what you're talking about with the prop valve plumbing. but that may be a contributing factor. on the early cars i generally run it direct to/from a distribution block and then only use a prop valve in the rear.

also, if the brakes haven't bedded in the stopping power can be less than what its full potential should be.

 

[str12-340]

following

 

[63Tigerlily]

a lot of this hinges on the master cylinder bore size. do you know what you wound up with?

i don't know exactly what you're talking about with the prop valve plumbing. but that may be a contributing factor. on the early cars i generally run it direct from a distribution block and then only use a prop valve in the rear.

also, if the brakes haven't bedded in the stopping power can be less than what its full potential should be.

Unfortunately, I have no idea. RockAuto had 2 listed, one drum/drum and one disc/drum. I bought the disc/drum, but I didn't double check anything on it. I'll see if I can get under there and measure the bore, and take a pick of the valve. (I plan on fixing the valve plumbing eventually, but didn't think it would have any big affect).

 

[junkyardhero]

rock should list the bore size in their specs, which may save you contorting yourself under the dash

 

[Professor Fate]

What bore size master did you get? This one? (the bore size is usually given under the "info" button)
More Information for DYNAMIC FRICTION 35540016
Factory used a 1 1/32 master on manual disc cars, power disc used a 15/16 master- which is a popular mod for manual cars and gives a much firmer "feel". More Information for DYNAMIC FRICTION 35540014
One thing you may also need to do is get an adjustable brake pushrod, as there sometimes seems to be a need on the early As to have a bit longer of a rod.
Mopar '62 -'70 Adjustable Master Cylinder Push Rod

 

[Dana67Dart]

On rock auto they list the specs of the ms including bore.

Your description sounds like you have a lot of air still in the system.

Look in your FSM for the proper routing.

Did dr diff recommend a stock combo valve

 

[Dana67Dart]

I am currently running manual, no booster

Meaning you do not have a booster on the car correct?

For those of you driving factory manual disc A-bodies from the seventies, what do the brakes feel like?

Manual stock KH front disk and 10x1.75 rear drums. The brakes work fine but take more leg pressure then our modern cars then we accustomed to

 

[harrisonm]

I totally agree that the Master Cylinder could be the culprit. However, you did not mention whether you changed the front brake hoses. Old brake hoses can cause a slightly spongy pedal. I assume you swapped them out. Just asking.

 

[65TerrorCuda]

65 barracuda, 11.75 79? cordoba rotors, pin calipers (72 Barracuda iirc), 10x1.75 rear drums, stops like crazy with very little pedal pressure. Manual dual circuit mc. New everything except spindles. Fresh shoes out back made a good difference. Adjustable prop valve but it's full open and not doing anything.

The original 10" front drums were great until they would lock, which I found harder to modulate vs discs.

 

[67Dart273]

You need to draw out or post photos of the tubing routing you used. Normally, the prop valve is "it" on these cars. That is the normal combo valve does NOT have a separate distro block. The REAR port on the master goes to the part of the prop valve that feeds the FRONT brakes.

1970 - 1976 Mopar Combination Valve Autopsy | Muscle Car Research LLC

www.musclecarresearch.com

 

[67Dart273]

So far as bleediing them make absolutely sure you have the calipers on the sides of the car so that the bleeder ports are at the top. This is a common problem what with guys switcing spindles, and such. Also, adjust the rears up tight to bleed, and see if that helps the pedal. Then back them off and recheck the pedal You WILL have more travel with manual brakes than with boosted

 

[MoparLeo]

Since we have no pictures and just a minimum description of your brake setup a lot of assumptions will happen. Limited info =limited opinions.
Using a 1976 Plymouth FSM would be the first reference source to use for the brake system.
All the brake specs will be there. M/c bore size, wheel cylinder bore size, caliper piston size etc...
If you sourced the front brake system from a 76 A-body did you also source the combi and prop valve as well.
It is a complete system.
Brake pedal travel and leverage are also different on drum/disc cars.
And as far as expecting "modern car brake performance" from a 1960's designed braking system, 50-60 years old. It will not do that. Modern cars almost all have an ABS braking system which is a computer controlled, high pressure pump system.
Just like you can't expect a 1976 6 cylinder engine to perform like a 2025 6 cylinder engine of today.
The most you could expect is to duplicate 1976 brake efficiency.
Older cars also sometimes had different brake pedal arms with different pivot points to increase or decrease pedal effort, as well as different length push rods.
You have to duplicate every item front to back to duplicate the system.
More to brakes then just hanging parts.

 

[340sFastback]

You will need to know master cylinder bore size, front caliper piston bore size, and rear wheel cylinder sizes. All of that will effect pedal travel and performance of the brakes. Most people use a 15/16 bore master cylinder for manual brakes.

 

[Oldmanmopar]

All lines must go through the proportioning valve for them to work properly. maximum master piston size is 1 inch. I could put someone up against the windshield at over 100 mph. I ran both iron and aluminum master and both worked the same..

http://www.moparts.org/Tech/MoparAction/Master_Cylinder.pdf

 

[townsend]

Keep in mind that 1976 calipers were a one year only for abodies.
Caliper bore size was 2.75 inches.

 

[jos51700]

Did you bench bleed the master cylinder?

 

[Professor Fate]

As a last resort-
I had issues getting a firm pedal on my '66.
Same deal, all new parts. 15/16 master. It would bench bleed okay, but would not generate enough pressure to work effectively.
On a lark, I disassembled the new master, and found that the entire secondary piston was installed backwards...

Reassembled it correctly, re-bench bled it and it's all good now.
Your findings may vary, but in the end don't forget the old adage: "New doesn't necessarily mean good".

 

[dogbreath]

Try the AutoZone NM1571 master cylinder . The one I received is all aluminum and has a 15/16 bore contrary to their website which says 1-1/32 bore , its a very nice unit , I paid $58 but I see they are $73 now.

 

[Murray]

Any "proportioning" is for rear brakes only. "Distributing" is for all brakes.

 

[72bluNblu]

The only thing I did (due to a rounded off line nut) is that my proportining valve front circuit is connected through one front port to the factory distribution block, rather than the two front circuits being connected directly to the proportioning valve. I also understand that the master listed for disc brake fronts may very well NOT be that, as I purchased it from RockAuto.

Pretty sure you're not proportioning the rear brakes properly, which could absolutely cause a long, soft pedal. Depending on the actual plumbing, you may not be getting any rear brakes at all. Or maybe not any fronts, who knows without seeing how you've hooked the combination and distribution blocks to the actual brake lines. You should have one or the other for the combination valve and distribution block, not both.

The factory combination valve uses a piston that slides back and forth. It can be recentered by a specific brake bleeding procedure. If the front brake circuit isn't hooked up, well, it's not proportioning. And how it's hooked up will determine what's not working.

 

[AJ/FormS]

I just finished putting '76 Scamp front disks on my 63 dart and a 75 dart rearend with the 10X2.5" shoes. I told doctor diff my setup (essentially a factory disc brake car) and he recommended the proportioning valve I would need. I purchased a new dual master reservoir listed for a 76 scamp with disc front brakes. Everything is in, bench bled, and bled with no more air coming out. I am currently running manual, no booster.

The brakes do not toss me into the windshield as I expected. The pedal is a little spongy, with quite a bot of travel. They sem to stop OK, but the pedal is not hard. It pretty much feels like the 9" drum system I replaced them for. The only thing I did (due to a rounded off line nut) is that my proportining valve front circuit is connected through one front port to the factory distribution block, rather than the two front circuits being connected directly to the proportioning valve. I also understand that the master listed for disc brake fronts may very well NOT be that, as I purchased it from RockAuto.

For those of you driving factory manual disc A-bodies from the seventies, what do the brakes feel like? Has anyone experienced this before? I specifically chose factory components to avoid the all-to-common disappointment of aftermarket conversions.

AS to your question;
I run the KH 4-piston set-up on mine with 10x 2s on the back, and braided lines on the front,
and with the Combination valve shown in Post-21,
and with a 15/16ths Dual M/C,
and with an F-body booster.
and I gutted that P-valve section, for use with 295/50-15s.
My pedal is high and hard, and the Barracuda is, bar none, the best braking car that I have ever had.

My guess is threefold;
1) the rear shoes are NOT adjusted tight enough, or
2) the inter-chamber in the M/C still has air in it. That would be the circled-in-red section shown in Post-18.
That chamber having air in it, is sortof a common occurrance/complaint here on FABO. Bench-bleeding it, should have eliminated that.
3) one thing rarely talked about, is rust deposits in the rear brake line, that present as a restriction. When the fluid passes from the high pressure side of the restriction, to the lower pressure side, it sometimes forms bubbles..... which then collect on the low-pressure side, giving a spongy feeling. Left alone over time, they seem to resorb into the fluid. But the next time you stab the pedal, there they are again. When all else fails, I cut out the lowest part of the line, and try it again. Sometimes I have to replace the entire line. But, more often than not, it's the rear flex-line that is the problem.

FYI;
I like the 15/16th M/C for it's modulation ability with my combo of a High Cylinder-Pressure 367 and manual trans. The 15/16ths requires more pedal-travel to do the job, and my left leg is used to clutching, which also takes a lot of travel, and I do a fair amount of left-foot on-the-gas braking.

 

[Hyper_pak]

Keep in mind that 1976 calipers were a one year only for abodies.
Caliper bore size was 2.75 inches.

What make a 76 unique?
Bore size?

 

[junkyardhero]

What make a 76 unique?
Bore size?

correct. bigger piston.

 

[RustyRatRod]

AS to your question;
I run the KH 4-piston set-up on mine with 10x 2s on the back, and braided lines on the front,
and with the Combination valve shown in Post-21,
and with a 15/16ths Dual M/C,
and with an F-body booster.
and I gutted that P-valve section, for use with 295/50-15s.
My pedal is high and hard, and the Barracuda is, bar none, the best braking car that I have ever had.

My guess is threefold;
1) the rear shoes are NOT adjusted tight enough, or
2) the inter-chamber in the M/C still has air in it. That would be the circled-in-red section shown in Post-18.
That chamber having air in it, is sortof a common occurrance/complaint here on FABO. Bench-bleeding it, should have eliminated that.
3) one thing rarely talked about, is rust deposits in the rear brake line, that present as a restriction. When the fluid passes from the high pressure side of the restriction, to the lower pressure side, it sometimes forms bubbles..... which then collect on the low-pressure side, giving a spongy feeling. Left alone over time, they seem to resorb into the fluid. But the next time you stab the pedal, there they are again. When all else fails, I cut out the lowest part of the line, and try it again. Sometimes I have to replace the entire line. But, more often than not, it's the rear flex-line that is the problem.

FYI;
I like the 15/16th M/C for it's modulation ability with my combo of a High Cylinder-Pressure 367 and manual trans. The 15/16ths requires more pedal-travel to do the job, and my left leg is used to clutching, which also takes a lot of travel, and I do a fair amount of left-foot on-the-gas braking.

Good grief.