why dont rear discs work

[charlesh]

i changed rear to a 8.5 with disc brakes,but i cannot get presssure to rear brakes. what am i doing wrong?:::

[cynical]

check the proportionang valve make sure it is set up for disk brakes

[spacecadet]

Along with the disk brakes for the rear, the proportioning valve and the master should have been changed. That is assuming that the truck orignially had drums in the back.

Space.

[charlesh]

it did have drum brakes,i will change both those items. thanks

[Petro]

Neither need to be changed actually...Probably just need to bleed more...You can put
C6 Z06 brakes on all the way around without changing master or combo valve....

[harleyboy]

no they will need to be changed due to the difference in pressure needed for the disks the drum take less pressure to work.

[harleyboy]

heres the page for the cylinder and valves

[o07wray]

Harley is right... proportioning valve is set up for less volume but more pressure for the rear if its a drum wheel cylinder. with a standard balance of 60% front, 40% rear.

With disc brake caliper.. it holds twice or more the volume of a wheel cylinder for drums, and only needs 1/2 pressure due to the surface area of the piston in the caliper is much larger than the wheel cylinder piston for drums...

same thing for the master cly,,, it has 2 differernt size pistons attached to the single shaft. one was smaller for the rear wheels.. ( for the same reasons as above ) and the front has a larger piston.

A 4 wheel disc brake master cly will have equal size pistons...

[XTEN]

I didnt use rear disc when I put my 9' in,but when I was ordering parts from Moser,their kit had "Explorer type?"calipers.The guy I talked to said you might wanna get an adjustable proportioning valve,but it didnt come with the kit.He said the master would be ok,but maybe their calipers had different size pistons.Are you getting fluid out of them when you bleed them?

[charlesh]

thanks guys,your info is very helpful, i have ordered a new disc brake master cylinder and proportioning valve,my whole ext cab s-10 is nearing completion and i could not have done it without the input from this site. thanks for all of your help--but i might still need some more help,. i will post pics when complete. here are some specs/solid black with purple ghost flames/383 c.i. with 550/550 lift hyd cam. serpentine belts with all power and air/350 trans with 3500 stall/8.5 rear with full spool and 3.55 gears(disc all around I hope)c-clip eliminators.roll cage/fuel cell/manual and electric pump/weld wheels/corvette radiator/with mods to radiator support/manual fan/and i am sure there is more and only taken a year and two months so far.