brakes question, need some tech. help

So, I rebuilt the brakes on my 3/4 ton truck years ago and they have performed pretty decently ever since. Except one issue. That is that when I had the drums turned, I asked the numbnuts to just hit it lightly and get most of the score lines out and true it up. Well, they came back fully baked and cut to the bone and perfectly smooth of course. Only problem was they cut them past max. spec. I found this out after I tried to fit the relined shoes on and the arch didn't match up, and they both weren't even the same. So, being a hard to get drum, too bad, too late, I had to make due. So, I had to send the shoes back with hand measurements off the drums so they could custom cut the brake linings to match. Worked ok in the end, but also only when the brake adjuster was already at its limit.

So, now after some years and some brake wear later, the clearance distance between the linings and drum surface is a lot more and now I have to always double pump the brakes to get them to engage. It is not an air in the line problem, I'm pretty sure it is the brakes baking off a lot and then i have to double pump them to get them hit the drums.

So, my question is, does there need to be a residual valve in the line to keep the brakes from backing off so much? I don't think mine has one of those. I was never sure what that was, but seem to recall maybe that is the purpose of those. I didn't really need that at first, but I'm guessing maybe now I do?

I know the real solution is to just get replacement drums and start over. I did actually end up finding what I think is a good set to eventually work with. But, I'm not quite ready to go through the whole brakes again quite yet and swap it all out and redo the brake pads.

I'm thinking maybe I can put one of these residual valves in line to fix that problem until the brakes really have to be redone anyway.

Yes, as you can tell, I'm no brake expert. But, any solid info. you can give me on the residual valves, etc, would be great. Thanks.

 

<big>if you have a 3/4 ton truck</big>​

use wagner f3600 or bendix bd60102 for front drums
use wagner f3620 or bendix bd60113 for the rear drums

also if you have a dana 60 rear axle it has been found that a napa part number 640-1008 brake drum will work.

 

If your master cylinder is stock, it should have an internal residual valve at the end of the bore.
A residual will not keep the brakes from backing off too much, but it will "tighten" the system and provide a more solid pedal because of the low pressure trapped fluid.

 

You may also be at the maximum stroke that your wheel cylinders can provide. There are several reasons why there are limits set by the factory for cutting drums and rotors and one of them is for the reason I mention.

 

Seems to me if you had to keep getting by, some kind of spacer would be in order to bring the pistons back in so you don't have to double-pump the brakes to stop.

Or, as noted, another drum and the next time you need them turned, tell them if they're turned more than you ask for you ain't paying for it.

 

When turning drums the machinist will turn them until they are completely true. No more, no less. If they don't clean up within spec, they need to be replaced. Do the job right and replace the drums and shoes. BTW the shoes are cam ground, and the arc will be not be the same as the drum diameter. There needs to be heel and toe clearance between the shoe and drum.