The master cylinder in my LeMons-racing Dart has finally crapped out, so I'm replacing it. After doing a lot of searching around on RockAuto, I found that I was apparently using the 1" bore MC specified for 67-70 A-bodies with manual disc brakes. A lot of forum cruising has led me to think I should actually switch to the 1-1/32" bore MC from later A-bodies to achieve a firmer pedal feel with later-model 2.75"-diameter single-piston M-body calipers. (Larger bore master cylinder = shorter pedal travel, greater effort.) But that's not what I came here to talk about. What I wanted to talk about was some other weird $#!+ I noticed when searching through the RockAuto parts bin.

First of all, I'd read in many places that front disc/rear drum MCs have a larger (rear) reservoir feeding the front brakes while drum/drum MCs have equal-size reservoirs. That seems to be true for most of the MCs I've looked at, but not all. The MC I just pulled off my car has equal-length reservoirs. I thought I might have installed a drum/drum master back in the day, but this part on RockAuto looks identical to mine, cover and all, and is specified for 67-70 A-bodies with manual front discs. (Granted, the POWER disc/drum master does have a larger rear reservoir. Go figure.) This wouldn't be the first time I've seen a bad listing on RockAuto, but I've been using one just like this for 3 years of road racing with no apparent bias issues. What kind of symptoms would you see if you accidentally used a drum/drum MC on a disc/drum setup?

Another thing. So in 1971, the manual disc/drum MC bore size was increased from 1" to 1-1/32". But why is there another change in the parts listing between 72 and 73? The bore diameter didn't change, but all the part numbers for front disc/rear drum are different across that year change. For example, the Raybestos part number changes from MC36426 to MC36412. What's the difference?

_________________
Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

I'll answer the reservoir part.....

The reservoir size doesn't matter if you are religious about checking the fluid level.

The larger reservoir is just so that as the calipers self adjust the fluid level doesn't drop too far and you suck air......

Constantly check fluid and you'll be fine.

Also on the small reservoir remove fluid when changing pads and pushing the pistons back into the caliper........

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Agreed.
That is correct.
Explanation is very simple: This is not 1973 any more. We're fortunate to be able to buy new parts that are vaguely kinda almost sorta workably close to original for A-body Mopars. There's no money in it; the only reason why the aftermarket still bothers to cater for us at all is to be able to entice contracts away from their competitors on grounds of greater application coverage. Most of those companies long ago fired or retired their last person who knew anything about parts for cars anywhere near as old as ours, and all the application cattledogs are outsourced. That's why the same kinds of "dammit!" stories come up again and again and again: "I bought a new [part name] for my [year, model, engine] but when I got it home it's nothing like the original and it won't even fit! But when I get on RockAuto and Amazon, it says it's correct just like the guy at O'Reilly said!".

The master cylinder you're looking at is not a disc/drum master. It is a drum/drum master. The correct '67-'70 disc/drum master, with or without power booster, has a 1" bore and a large rear reservoir serving the front discs and a small front reservoir serving the rear drums.
It's not a bias issue so much as it is a safety issue with regard to fluid reserve.
Because expanders were added to the rear wheel cylinder seals, theoretically obviating the need for the residual pressure valve in the drum brake fluid outlet of the master cylinder.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

Awesome. Thanks for the info, guys. I think I'll go with the 71-72 part, for belt-and-suspenders safety on the rear drum residual pressure. Unfortunately, it appears that neither of those Raybestos part numbers are currently available on RockAuto, but there is a new Centric part available. The interchange years listed are kinda weird, they go to 72 on some cars and 76 on others. Shouldn't be a big deal; worst case, it just won't have the residual pressure valve for the rear.

So... if I've been using an early drum/drum master cylinder, it probably had provisions for residual pressure on both circuits, right? I may have finally figured out my front pad-dragging problem...

_________________
Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

"Ships in 1-2 months" on Amazon. Guessing we might be nearing the end of the line for this part.
I think maybe so!

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

What's wrong with using the '73+ master cylinder that was used with the single-piston front discs?

_________________
Joshua

I agree with Josh.

I ran a 73+ MC with late discs and 10" drum....then later switched to rear discs. I never had any bias trouble and the pedal effort was perfect. It bolted right on to my 65 .

_________________
Yeah....Im the one who destroyed this rare, vintage automobile.....

73+ master cylinder would be fine with modern wheel cylinders, which I did replace when I did the axle swap. I just went with the 71-72 part because it might (maybe) have the residual pressure valve for the rear drums. I don't need this, but decided it would be nice to have just in case the rear wheel cylinders leak.

That being said, the actual Centric part I purchased is listed for 71-76 A-bodies on RockAuto, so it may be an even mooter point than it was.

_________________
Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

My master cylinder in my 70 dart w/ 225 is leaking. It is manual drums. I want to convert to power disc eventually but it's probably a year away on the schedule. Is there ano MC that will work for both?

I'm wondering the same thing?

_________________
1966 Dodge Coronet Deluxe 2dr sedan
1966 Plymouth Belvedere II 2dr hdtp
Both slant 6 cars

This one or this one will work fine for the drum brakes and the eventual disc brakes.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

As a follow-up, the Centric 130.63019 Master Cylinder I bought from RockAuto back in August 2016 ended up being garbage. We chased potential fluid leaks and/or air in the system for two whole races, trying to cure an unpredictably soft brake pedal. I suspected the remanufactured front calipers we put on at the same time were the problem, but it turns out our new master cylinder was leaking past the seals.

Just goes to show that while "reman" parts are frequently garbage, "new" parts are not necessarily a guarantee of quality either.

I'd probably splurge and buy the Raybestos unit, if you can find one. MC36426 appears to also be available on RockAuto at the moment.

_________________
Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing