Residual pressure valve in master cylinder...

[sled_dog]

Alright, I am not a brakes genius, but in school we learned that a master cylinder has a residual pressure valve when equipped with drum brakes. Namely it holds pressure in the line to the drum brakes. Teachers claimed if converting to Disc you NEED to remove this, but I've never heard of this before they told me. So do K series rigs not have this or what?

 

[sandawgk5]

From what I understand they are only required if the master cylinder is below the height of the wheel cylineders. Much like in street rods where the master cylinder is below the floor board. I believe in out rigs the master cylinder is high enough above the wheel cylinders that there is adequate pressure to not need them. That is what I think I read if I am wrong please let me know.

 

[btolenti]

I've heard similar as well. Some people have had trouble when swapping from rear drums to discs, and have had to fiddle with the stock proportioning valve. I believe what they do is remove the stock components inside the proportioning valve, and plumb in an aftermarket unit to give you more adjustability....

http://coloradok5.com/forums/showthread.php?t=144961

 

[sled_dog]

maybe thats why you have to adjust our rear brakes some. The residual valve is in the master cylinder itself(at the output), so I was taught. Hmm, I wonder if its there and would help braking if removed.

 

[dyeager535]

I've seen what you are referring to, but I'm not at my manual to take a look.

Already been said before, but check out the Impala SS stuff: GM used the same proportioning valve between disk/disk and disk/drum in those applications. The only link I've ever found about modifying it for disk/disk was to b-body.org(?) and it had/has been dead for a long time. It was a modification to the stock proportioning valve which involved removing something from what I could gather from other talk about it.

 

[btolenti]

I went through a similar problem with my 68 Buick GS. I swapped out the rear for discs...actually a 9" with a disc conversion. I talked with Stainless Steel Brakes, and they said I would have to change my master cylinder because there were 2 different ones. 1 made for the disc/drum brakes, and 1 for disc/disc. Basically, the proportioning valve on my car just splits the brake pressure to the two front brakes and proportions the brake pressure to the rear....a pass through with no residual valves.

Then I have heard on here that the proportioning valves on our trucks sometimes (not always or even most of the time) need to be disabled, and an aftermarket one plumbed in.

I would go ahead and convert to discs in the rear, and try it out before you plumb in the aftermarket valve. The stock setup seems to work for 90% of the people.

Check out the bottom of this article. It goes into detail....

http://coloradok5.com/atrondiskbrakes.shtml