Ok,
My car was changed to the dual reservoir brake system using the 67 setup. Its manual drum brakes

Issue is I have a brake fluid leak I cant get to stop. leaks form under the cap. ( ive changed cap and gasket , and it still leaks a little.

I have decided to just buy a new master cylinder.
Question is,
I have been thinking about converting to disc brakes at some point in the near future and am wondering if I just buy a disc brake master now, is it compatible with my drum brakes till I actually convert it to disc brakes

OR if I buy a drum brake master cylinder can I retain it when / if I convert to disc brakes. Im looking at one of these new master cylinders at the moment
I am thinking conversion will not happen for a year or so ( I have other fish to fry before making that swap )

I think the biggest risk in using a drum MC on a disc car, is that their smaller sized primary reservoir might run low on fluid as the pads wear.
On a car that’s not driven a ton and whose owner checks and replenishes fluids often, that’s not likely to happen.
The B body disc MCs of that era were 1-1/8” bore and the drum MCs 1”, you may find the pedal feel and grabiness of the brakes not to your liking, but without trying it, it’s hard to say for sure.

really the only difference is that reservoir size. While the bore certainly could be different between what you have and what you get, that could happen regardless and really is something you can control by ordering a different year or model of master.

So that being said, it should not matter if you go ahead and get a disc master now.

Bore size is a different discussion, and I found I had to decrease the bore size to get good braking power. early small diameter boosters were dual diaphram for disc cars vs single for drum cars.
they generally don't provide enough boost.

If you go disc you really should go with Power Disc which means you really need an offset bracket. You are into changing spindles and such if you do this right and you certainly do not want the 67-69 hard to find and expensive set up. You would want 70-72 or go with the late model set up. You really just start to lose originality. Frankly even if they work those GM booster folks use just ruin the look of a hemi engine bay in my opinion. I believe drums set up is actually lighter also. Your maintenance work is easier with the manual drum set up. Just get a new equivalent MC.

My plan is use one of the modern kits which I believe is based on A or later F body setups.
Under hood IF i go power will be with a reasonable facsimile looking Mopar booster and master cylinder assy. with the offset booster bracket
Those GM looking bail wire master cylinder kits will NOT be on my car

In all honesty
If I can get away with the converting to disc brakes and keep the stock drum brake master cylinder is my preference. That way underhood would still look the same as today . I have ZERO interest to going backwards to the single pot master cylinder,,,,,,, EVER
I have no problem with cking the fluid level on a regular basis as pads were what so ever

A buddy of mine locally converted his 68 Roadrunner to manual disc brakes,,,,,, but he went later model plastic reservoir master cyl ( that master cyl is out of the question for me ) and he says his cars stops perfect

on a side note. My car stops fine. its the off chance I get into a driving situation of brake fade is why Im considering the change to disc

As an example, about a month back we drove up Mt Lemon mountain hiway which is about 25 miles ,,uphill, which equals 25 miles back downhill. And when we got the the bottom I could tell the brakes were hot as it did not want to stop as well as usual

I know some guys swear by manual disc, but drums are self actuating and disc are not. I know they sell the remake of the hemi type with no hemi booster stud pattern and offset. That may be the way to go, but a valve cover removal is more work. I guess if it was my car I would try a new manual drum set up to stop the leak. Have the original rebuilt and resurface seal area. Go from there.

Drum masters have a residual valve build in to maintain a few psi of line pressure when the pedal is released. When I converted my power wagon over I kept the drum master. But I used I screw to pull the brass brake line seat, so that I could remove the valve that was behind it.

I prefer the feel of manual brakes; the resistance is easier to modulate.
I suppose power has an advantage when you need to lock 'em up RIGHT NOW, but then it's the tires and trajectory that tend to dictate the outcome.

Im aware of the drum brakes self generating force by design .

I don't have a problem with having to pull the booster assy off once a year for spark plug or valve adjustment. if I have to go that direction.
I just like the clean uncluttered no option look of the drum master


I have to buy a master cylinder and was thinking just buy another drum unit. UNLESS I HAVE to convert to a disc brake unit and/or power assist I will .


EDIT,
I just ordered a new drum brake master cylinder. I think I will try to use it with the disc brakes when that change happens.
If it doesn't work or feel right. I 'll update to a disc unit and probably go with power assist .
But like mentioned,,,,that change is at least a year out ,,maybe more . I have to get this fluid leak fixed soon,