Which master cylinder for 4 wheel disk brakes

[nm9stheham]

On older cars, not really sure what they did. Maybe they are 50/50.

On current stuff...well, they're showing the master cylinder bias on basically all of them, short of a few diagonal split cars. I work on ESC modules and we get this information direct from the OEMs. Of course we can't share this information directly. There are even diagonal split cars with not exactly 50/50 splits, which couldn't happen any other way. I believe that for a larger split they are also using orifices.

Since the secondary piston floats, in order for it to move the pressure on the other side (being applied by the primary piston) must exceed both the pressure in the secondary bore AND the spring force of the secondary return spring in order to move. I'm not sure exactly how strong the springs are

The only time the primary piston actually acts directly on the secondary piston is when the primary circuit has a leak.

With light springs, which is what I recall on older stuff (it has been quite a while since I did a MC kit), they really would not enter the picture, and the bias should be 50-50 (equal line pressures from the MC F and R). Buuuut now I gotta think.....if the secondary piston spring WAS pretty darned strong, then it seems like it would indeed bias the line pressures to the front. You might need a stop pin to put in the bore after assembling to give the secondary piston a stop. (I recall that some old MC's actually did have a stop pin.)

 

[BillGrissom]

In our cars, if the F & R circuit pressures aren't almost equal, the "pressure imbalance switch" will light the "brake warning" dash lamp. That switch is integral with the "combo valve" in disk-drum cars or the "distribution block" in drum-drum cars (~1968+). My 1996 Voyager appears to have a functionally equivalent switch integral with its ABS block, and it is has X-split plumbing.

The slides I linked show an MC w/ 2 pistons with a spring in-between, but doesn't clearly describe the function of the springs (to me). I think the purpose is that if the R piston sticks in the bore, the F piston will still retract (via its spring) to be ready for the next brake application. But, as stated by others, any spring force will cause some pressure bias.

The photo is of the Breeze MC rebuild kit I have (mine is ABS). It was actually a safety recall (#687), w/ more info here:
"ww2.justanswer.com/uploads/heavychevy396/2010-10-05_010720_1996_Dodge_Stratus_recall.pdf"
My kit is actually just an upgraded "primary piston". Note the 2 piston seals. The spring acts on the secondary piston, as in our 1968-78 Mopar MC's.

The 7/8" piston may be over-kill for my 64 Valiant w/ manual brakes and 9" drums. I haven't driven it thus, so can't comment. It may be perfect if I ever change to front disks and keep the MC manual. I use the same MC on my 1965 Dart, but with a Breeze power booster. I don't have significant pedal motion there, but the lever in the firewall brackets makes pedal motion less (~40%). That is why power brake cars often have smaller MC bores (confuses the heck out of some here). I even use silicone brake fluid that many claim is "spongy" (don't notice such). More commonly, a long pedal travel is due to air bubbles in the system. For sure in my Dart the small MC bore, booster, and 10" drums makes the brakes "authoritative", but I have driven the car a bit and it doesn't throw me thru the windshield. It will be about perfect w/ front disks some day. I even used the same MC on my 65 Newport w/ factory booster and 11"x3" front drums. It stops well, and I appreciate the smaller bore since when a booster failed (factory 1" MC) I had to use both feet to stop and the 4000 lb beast felt like a run-away train.