4otF said:
Nope. The larger port is for the disks. Less pressure than drums but much more volume. Usually the reservoir toward the firewall is for the front brakes and the front reservoir is for the rear brakes.
Hah. You guys are trying to be logical, and the factory engineers obviously didn't care about logic. I have a big pile of cylinder cores from the major manufacturers, and 10 years experience looking at other people's cylinders. I've never been able to detect a pattern. Some have the large reservoir to the front, some to the rear. Some have the large port to the front, some to the rear. Some have the large port at the same end as the large reservoir, some the other way. One GM cylinder in my bonepile, probably from a late '70s-early '80s 1/2 ton, has the proportioning valve still plumbed up to it. The small port has a 1/4" line and the large port, which in this cylinder is for the front discs, has a 3/16" line. So...in my experience, which is considerable, you can't draw any meaningful conclusions from the port sizes or the line sizes.
If it's a correct disc/drum master, the fluid reservoirs will be of different sizes. Large reservoir for the disc brakes, small reservoir for the drum brakes, and you can take that to the bank.
Note that with rare exception the pressure is the same for front and rear because the primary and secondary master cylinder pistons are the same size. The only way to get different pressures out of the master is to make the pistons different sizes. A few mid '60s to early '70s American vehicles had stepped master cylinders, mainly Ramblers and IH Scouts, but most stepped cylinders are made by Girling and are in foreign cars.
Disc caliper pistons are much larger than wheel cylinder pistons, but the pads stay quite close to the disc when the brakes aren't applied--so close that disc brakes run much hotter than drum brakes. This means that the volume required for activation of disc calipers and drum cylinders differs so slightly as to be insignificant, certainly not by enough to require larger lines or ports for disc brakes. The reason the disc reservoir is larger is that the disc brakes self-adjust by displacement of the piston as the pads wear, thus increasing the volume behind the piston, and the reservoir is supposed to contain enough fluid to last the life of the pads even if it is never refilled. So the additional volume is needed over the life of the pads, not at each application of the brakes.
Joe
