Master Cylinder Question

[mopowers]

I was bench bleeding my master cylinder today, which originally came on a late model Dakota I believe. I noticed the front port starts pushes fluid slightly before the rear port starts, though it stops quite a ways before the rear port stops pushing fluid.

Assuming the front master cylinder port is for the rear brakes and the rear master cylinder port is for the front brakes, is it normal for the rear brakes to start actuating before the front brakes start???

 

[Kern Dog]

Let me start by stating that I do not know for certain BUT I recall reading the following:
Some master cylinders have the REAR brakes fed from the front reservoir, some feed the rear brakes from the REAR reservoir.
Traditional Mopars all seem to be reversed from what makes sense. FRONT reservoir feds the REAR brakes, REAR reservoir feeds the FRONT brakes.
I posted about this a few years back and a FABO member posted up a picture of some aftermarket master cylinder that looked like these:

...BUT the front reservoir was supposed to feed the FRONT brakes.
I've read that some master cylinders have a stepped bore, I suspect that these are for FRONT wheel drive cars with a huge front weight bias.
I have a fuzzy memory of something that I thought I read in Mopar Action magazine...
Rick Ehrenberg wrote something years ago and what little that I remember from it was that the rear drum brakes need some pressure to overcome the shoe springs but overall, the drum brakes require less pressure to work due to the self energizing action where the front shoe digs in, forcing the rear shoe to follow....an action that reduces the need for additional pedal effort. This, in a way, makes drum brakes non linear in how they operate. 20 lbs of pedal pressure may give 100 lbs of brake pressure while 40 lbs of pressure may give 300 lbs of brake pressure. This is just an example, not a hard fact.
Disc brake calipers ARE linear.
Because of this, the rear gets pressure first to overcome the springs that connect the shoes together but then the TOTAL pressure is limited (via the proportioning valve) to prevent wheel lockup.
In short, according to Ehrenberg, the rear gets pressure first but then stops building pressure once the proportioning valve does it's job.
Hey....If you want to test different master cylinders, give me a call.

I like to experiment with this stuff....

 

[mopowers]

Let me start by stating that I do not know for certain BUT I recall reading the following:
Some master cylinders have the REAR brakes fed from the front reservoir, some feed the rear brakes from the REAR reservoir.
Traditional Mopars all seem to be reversed from what makes sense. FRONT reservoir feds the REAR brakes, REAR reservoir feeds the FRONT brakes.
I posted about this a few years back and a FABO member posted up a picture of some aftermarket master cylinder that looked like these:

View attachment 1716445658

View attachment 1716445660

...BUT the front reservoir was supposed to feed the FRONT brakes.
I've read that some master cylinders have a stepped bore, I suspect that these are for FRONT wheel drive cars with a huge front weight bias.
I have a fuzzy memory of something that I thought I read in Mopar Action magazine...
Rick Ehrenberg wrote something years ago and what little that I remember from it was that the rear drum brakes need some pressure to overcome the shoe springs but overall, the drum brakes require less pressure to work due to the self energizing action where the front shoe digs in, forcing the rear shoe to follow....an action that reduces the need for additional pedal effort. This, in a way, makes drum brakes non linear in how they operate. 20 lbs of pedal pressure may give 100 lbs of brake pressure while 40 lbs of pressure may give 300 lbs of brake pressure. This is just an example, not a hard fact.
Disc brake calipers ARE linear.
Because of this, the rear gets pressure first to overcome the springs that connect the shoes together but then the TOTAL pressure is limited (via the proportioning valve) to prevent wheel lockup.
In short, according to Ehrenberg, the rear gets pressure first but then stops building pressure once the proportioning valve does it's job.
Hey....If you want to test different master cylinders, give me a call.

View attachment 1716445662

View attachment 1716445663

View attachment 1716445664

I like to experiment with this stuff....

Thank you Greg. And thanks for the chat earlier. I actually found an old thread of yours that I believe answers my question. Master cylinder and brake line questions.

In that thread, a gentleman (1STMP) mentions that "Whatever front/rear set-up you run, wether it be drum/drum, disc/drum,or disc/disc, you'll want the rear to react first, if only for a fraction of a second. This reduces nose dive under hard braking situations. An adjustable proportioning valve allows you to dial this in to a finer reaction time."

I am running an adjustable proportioning valve in the rear line, so I guess it's normal for the rears to start applying before the fronts. Then I can simply dial in the amount of rear brake with the adjustable prop valve, and the fronts will apply for a longer amount of pedal travel since they require more pressure.

What got me thinking this was incorrect was the fact that my master cylinder has a "P" cast into the body on the front. I was assume this means the front port on the master cylinder is the "primary" port. I would think that meant the port was for the front, but what do I know?

I'm just curious what others say. Anyone know what the P port is for - front or rear?

 

[Bewy]

When the last of the 4 wheels receive pressure...is when the brakes will apply. It could be any one of the 4 wheels & a veeeeery small amount of time is involved.