I am getting the brake system finished up on my brother's 74 Duster. The car originally had nine inch manual drums all around. The car has been upgraded to manual disc brakes in front and 11 inch drums in back. On Dan's recommendation, I sized the rear drum wheel cylinders down to ones with a 3/4 inch bore since the 11 inch rear drums are freaking huge.

My question is what master cylinder should I buy? I have a proportioning valve that came with the disc brake swap parts bundle I bought, but I don't have the disc brake master cylinder.

Rockauto gives me several choices for disc brake master cylinders:

Wagner or Dorman brand 1 1/32" bore manual disc brake master cylinder,

or

Dorman brand 15/16" bore power disc brake master cylinder

I am thinking that the huge rear drums will be touchy enough, so I don't think I want to go with the smaller bore master cylinder, right? Or am I getting the hydraulic multiplication principle backwards (smaller bore = more force transferred, yes?)?

_________________
Casually looking for a Clifford hyperpak intake for cheap.

Would an adjustable proportioning valve make sense in this situation? Regardless of what master cylinder you get, that is.

Reed,
I think you have your HS Physics down…

Smaller bore master cylinder will produce same force at wheel cylinders, but require longer peddle travel with lighter foot pressure compared to a larger bore master cylinder shorter travel higher foot pressure. It’s all in the ratio of fluid displaced, and PSI applied.

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

A smaller bore will give you easier modulation, meaning it will be easier to control (and correct for) lock up and braking "threshold." Yes, you will have longer pedal travel. Yes, if you TROMP the brake pedal, then the smaller bore will make it lock more easily, but with any kind of foot finesse, you are better off with the smaller bore. Think about lever arms - the smaller bore is like having a longer lever arm to lift a weight or tighten a bolt - more control is possible.

I add an adj prop valve to all of my brake systems for fine tuning. It will not compensate for a large mismatch in the front/rear bias, so WC or caliper bores are the best way to make large changes and get close.

Lou

_________________
Home of Slant6-powered fun machines since 1988

OK- thanks. I will give the small bore master cylinder a try.

_________________
Casually looking for a Clifford hyperpak intake for cheap.

I've got a similar brake system. Manual disk brake front, with the 8" Ford rear off a V8 Maverick. The rear drums are 10", so the change in wheel cylinder will likely be right for your 11" set. The '74 brake master for the drums works well, with no rear lockup, and excellent results. I've flogged the brakes hard too. Pedal pressure is firm with great feedback.
Drum brake master is 15/16", the manual disk master is 1-1/32". So basically, you have the right master cylinder already.

That's in my '74 4 door Valiant. Didn't change the proportioning valve. Worked well at stock weight, and continues to perform flawlessly in lightened form, on the road radials, or on drag slicks and front runners. I've had no tendency for lock on front or rear prior to lockup on all four corners. After the mod and a bit of conditioning, I took it out and stomped the stuffing out of them to make sure it was going to behave itself. You should probably do likewise with the oversize drums out back.

I replaced all hardware when I put it together. New rotors and claws. New drums and wheel cylinders.

2¢

CJ

_________________
Part of Tyrde-Browne Racing

Well, that is good news CJ, since lack of funds prevents purchase of a new master cylinder right now! I guess we will bolt it all up as-is, bleed it, get it aligned, and see how it performs.

_________________
Casually looking for a Clifford hyperpak intake for cheap.