Disk/drum brake operation?

[nuclearbeef]

Short version of my question:
If rear drums are not adjusted properly, will this prevent the front disks from operating?


Long version:
Since I've owned my pig, my brakes require pumping to operate. First application of pedal goes all the way to the floor and the brakes just begin to bite. Releasing the pedal and re-applying brakes gets me a firm pedal and good braking.
So, after several years of driving it this way, I finally decided to bleed the front brakes yesterday. (Front, because I'm pretty sure the rears bite first when I hit the pedal the first time.) This did nothing to change the operation of the brakes.
I always thought that the front and rear systems were completely separated and that loosing the rear brakes would not cause the front to quit working. I guess I'm mistaken in this assumption?

(Yes, my next step is to open up the rear drums and fix/adjust as needed. Yes, I know I'm not very wise (massively stupid) for not fixing this earlier.)

I'm not looking for a tutorial on how to adjust the brakes; more of a lesson on how the system operates and how the pressure goes to each brake from the master cylinders.

 

[bobm]

70 percent of your braking is the front Drums or disk.

 

[nuclearbeef]

I just adjusted the rear drum cylinders and now have good, even braking on the first pedal application; but, I still don't understand why I had no front brakes on the first pedal application before adjusting the rears.

Why wouldn't I just loose the rear brake effectiveness if only the rears were out of adjustment? Why wouldn't hitting the pedal move the master cylinder for the front brakes; moving the fluid for the front brake system; moving the calipers; squeezing the disks and getting braking on the front... while simultaneously moving the master cylinder for the rears; moving the rear fluid; moving the rear wheel cylinders; but just not far enough to apply rear brakes because of the bad adjustment?

I thought that's why there was two separate hydraulic systems for the front and rear. Redundancy, in case you lost fluid to one or the other.

I'm obviously wrong, but I don't know why.

 

[bobm]

proportioning valve?

 

[nuclearbeef]

Well, I googled proportioning valve and found this:

"The metering valve compensates for this, making the drum brakes engage just before the disc brakes. The metering valve does not allow any pressure to the disc brakes until a threshold pressure has been reached. The threshold pressure is low compared to the maximum pressure in the braking system, so the drum brakes just barely engage before the disc brakes kick in."

Makes sense now.

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/master-brake2.htm

 

[billstoudt]

yeah proportioning valve when running disk drum combo for sure. for a very short period of time i had willwood disc in the front and stock drum in the rear i could never get it to stop as well as i thought it should. i ordered a proportioning valve from summit racing but in the mean time i bought a rear disc kit from SOR . once i got the rear discs in the rear and got them bled properly it stops better than my 2010 pathfinder. so personally i would skip the valve and pick up a rear disc kit. while your at it pick up a new master cylinder and your pig should stop on a dime.

 

[LAMBCRUSHER]

I think it just has more to do with the rears needing to be adjusted. if they aren't helping any, then 100% of your braking is coming from the fronts. And they will work with some effort at this for a while. Best to have the rears help, too, tho...