Hi friends. New member here. I have a 1961 Plymouth Belvedere sedan with the 225 slant six and manual drum brakes at all four corners. I want to upgrade to a dual-resevoir master cylinder. What year/model car has one that will swap in with the least amount of grief? Where might I find a quality new/rebuilt master cylinder? Also, I've heard that no proportioning valve is needed for this kind of early set-up. Any help with this swap would be greatly appreciated. Thanks in advance!

When I did the same on my 64 300 I just got a later, 4 bolt, dual chamber master cylinder and bolted it up. You will, obviously, need to plumb it.

I think I just matched bore diameters, but it's been 40 years.

i'm thinking the 4 bolt single pot masters share the same bolt pattern as the 67 up dual chamber masters.
as Sniper said, you will need to match the bore size and plumb it. remember though, the rear chamber goes to the front brakes, and the front chamber goes to the rears.
you may be able to use the original push rod, but i can't guarantee that.

The '61 MC has a 1 1/8" bore. If you use a '67 or later manual drum brake MC of the same bore, no prop valve is needed.

Do verify the pushrod length. You should have ~.030 clearance when the pedal is fully up.
If your brakes are working fine stay with the 1-1/8 bore. You can drop to a 1" bore to improve the braking a touch but it will require a little bit more pedal travel

The manual brake FL cars of that era have a pedal ratio of 8-1 where 6-1 is the norm. Smaller bore MC's will eat up a lot of pedal travel.

Most of the 1 1/8" dual pot MC's are for disc brakes and don't have a RPV for the front system...might be problem, might not.

We use only 70 C body masters Just make sure its drum drum If you go to discs then use disc master Dosent matter if power or manual brakes Kid ran it on his 62 300 all drum for 10 years at least My 65 has had one for 20 years.