upgrade master brake cylinder

-
Yes, it was one of the shop tools I used during my apprenticeship, back in the 60's.
It was okay.
I prefer the vacuum bleeder hooked to engine vac port, or handpump, whichever is timely and convenient.
 
The old style is easier.
No adapters, no fittings, pushrod fits without adjustment, no flare tool.
4 nuts, one pedal pivot bolt.
Bleed as nec.

Didn't say better, as they are all just a pump .

And we both have agreed for a many years, that the 15/16 bore is preferred by spirited driver's for it's "feel and modulation". (sound familiar?)

EDIT.
 
Last edited:
The bore size is important for the amount of effort you have to apply.
Smaller bore gives you more " mechanical advantage/leverage" requiring less effort, often used when eliminating the power booster, and often preferred by " spirited" drivers.
The "look" is up to you.
My car, I've had since new, and I'm keeping it original, - to some, that look isn't important, and may prefer the modern look.
Both pumps work exactly the same .
 
Has anyone with a factory '70 A body manual disc car installed the MC5621 and if so how much more pedal effort in your opinion?

Also does anyone who replaced their manual disc master with the DoctorDiff have some pics of the finished install?
 
mrhp said:
Has anyone with a factory '70 A body manual disc car installed the MC5621 and if so how much more pedal effort in your opinion?

Also does anyone who replaced their manual disc master with the DoctorDiff have some pics of the finished install?
Click to expand...

An 1-1/8" master will be a significant increase in pedal effort. That's even larger than the factory manual master cylinders which are 1-1/32". To me, even the 1-1/32" stock manual master results in a pretty hard pedal. Changing the stock master out for a 15/16" changes the pedal effort dramatically, although it does increase the amount of travel needed. The smaller bore results in a higher brake line pressure and a longer pedal stroke. The larger bore reduces line pressure and reduces the pedal stroke, but that means all the force has to be applied over a short pedal distance, which gives the high effort. There's no way I'd run a master cylinder bore that was larger than the factory 1-1/32" with the factory brakes, that will make the effort even higher and I already swap the stockers for 15/16" on all my cars.

Here's a picture of the DoctorDiff master cylinder in my Duster. The hydraulic clutch master cylinder is to the right of it and my adjustable prop valve is below it.
IMG_6565.jpeg
 
mrhp said:
Thanks for the science and the pic. Beautiful.
Click to expand...

I dunno about "beautiful", I was gonna include my usual "excuse my dirt" but forgot. It works great though!
 
72bluNblu said:
Have you even used a pressure bleeder? Makes vacuum bleeders seem stupid. I used vacuum bleeders for years and there’s no way I’d buy another one of those either.

If folks want to use the old cast iron master cylinders that’s fine, but let’s not pretend they’re easier or better.
Click to expand...
I'm thinking about converting my bone stock 1974 Duster with its original power front disc/rear drum brakes to the Mr. Diff aluminum and plastic MC referred to in this thread. How hard is this to do? Do the brake lines thread in correctly? Is there anything I need to do with the rod that comes out of the front of the vacuum booster? Thanks.
 
cruiser said:
I'm thinking about converting my bone stock 1974 Duster with its original power front disc/rear drum brakes to the Mr. Diff aluminum and plastic MC referred to in this thread. How hard is this to do? Do the brake lines thread in correctly? Is there anything I need to do with the rod that comes out of the front of the vacuum booster? Thanks.
Click to expand...

It’s pretty easy. You’ll need the 4 bolt to 2 bolt adapter and a manual brake push rod (or an adjustable push rod from DoctorDiff). The power brake push rods won’t work with the manual master cylinder. The thread size on the lines should be the same, but they won’t be the right length/shape because of the change in location of the master cylinder after you delete the booster. Depending on their condition you may be able to bend them to make them line up. The factory lines don’t like to bend though, and if they’re even a little rusty they won’t handle the bending necessary to get them to line up with the manual master cylinder. For my money I’d replace the lines from the master to the combo valve, just order them now assuming the originals won’t bend without kinking or leaking. Otherwise you get the job half done, have a problem, and have to wait for parts or bend your own lines mid install.

DoctorDiff sells everything you need for the swap.
 
72bluNblu said:
It’s pretty easy. You’ll need the 4 bolt to 2 bolt adapter and a manual brake push rod (or an adjustable push rod from DoctorDiff). The power brake push rods won’t work with the manual master cylinder. The thread size on the lines should be the same, but they won’t be the right length/shape because of the change in location of the master cylinder after you delete the booster. Depending on their condition you may be able to bend them to make them line up. The factory lines don’t like to bend though, and if they’re even a little rusty they won’t handle the bending necessary to get them to line up with the manual master cylinder. For my money I’d replace the lines from the master to the combo valve, just order them now assuming the originals won’t bend without kinking or leaking. Otherwise you get the job half done, have a problem, and have to wait for parts or bend your own lines mid install.

DoctorDiff sells everything you need for the swap.
Click to expand...
I'm not converting to manual brakes. I'm keeping the power booster. All I want to do is to remove the leaky existing factory MC (leaving the vacuum booster in place), and bolt on one of Dr. Diffs aluminum and plastic MCs. I'm thinking that all I need to do is unbolt and remove the old one, bolt the new one in its place, and bleed the system. Your thoughts?
 
cruiser said:
I'm not converting to manual brakes. I'm keeping the power booster. All I want to do is to remove the leaky existing factory MC (leaving the vacuum booster in place), and bolt on one of Dr. Diffs aluminum and plastic MCs. I'm thinking that all I need to do is unbolt and remove the old one, bolt the new one in its place, and bleed the system. Your thoughts?
Click to expand...

Sorry, I assumed you were ditching it. Hate those things. With a 15/16" master cylinder you don't need one and the brake feel and modulation is better without one.

Pretty sure you'll still need the 4 to 2 bolt adapter that DoctorDiff sells, so buy the whole kit not just the master cylinder. The push rods should be fine, but you might want to ask DoctorDiff because honestly I convert all of my factory power brake cars to manual brakes.
 
Sorry if this has been answered, I tried to read through most of the posts. I simply want to upgrade my single jar type master cylinder on my 63 to a dual reservoir. For now I'm keeping all the brakes drums and staying manual. I'm reading the 15/16 bore being preferred but that seems to be for front discs. What's a good one for drums? I'm looking at several options on the 2660809 one inch bore. Best price I found was at piratejack.net. Thanks!
 
You must log in or register to reply here.
-

Back
Top