My Rear Disc swap from drums observations

[John Cunningham]

I agree with the OP,, I did the same thing on my old 8,600 gvw squarebody K2500. It had the JD7 brakes, with a 14ff axle in back, but used vacuum assist.
I pulled the residual valve out of the master cylinder port that fed the rears, switched to a disc/disc proportioning valve, added a Willwood adjustable in the rear line, and the brakes went from ok to great.
You only had to move the pedal enough for the pistons in the master to close-off the compensating ports and the pedal became rock hard,, I think spending a lot of time in bleeding, along with a lot of fluid being used made the pedal hard,, their couldn't have been even a pin-head sized air bubble anywhere in the system.
You might still have some air,, many times a bunch of microscopic sized bubbles over time will gradually move up-hill until they "merge together" to form a bubble that gives you a softer pedal.
If the fluid column is absolutely 100% "solid" the pedal will be solid.

I have a question......would the residual valve that is probably in my master cyl only put keep pressure on the rear drums that previously existed or both front discs and rear drums? I never pulled it out but the thought entered my mind. I am not sure of how much pressure this valve leaves in the line and which brakes it affects. With the new rear discs it sounds like there is some residual pressure that is applied to the rear.......I do want to increase the rotor size and dual discs in the front before summer for towing purposes but not sure about the prop valve. I have used them years ago in a street machine but could not tell much. Its been awhile since I did that. I went out on some gravel to test it to see if the rears locked...they did not...so I left it alone never played with it after that.....

My pedal was solid and rock hard before and I really had to push hard on it to get some brake reaction. Now the pedal is not rock hard but neither is it mushy and I dont have to push as hard on the pedal to get it to stop either....

Thanks for the information

 

[R422b]

The residual presure valve keeps a small amount of pressure on the drums only, because the wheel cylinders tend to leak air in if the presure goes slightly negative like when the petal comes back up after a hard stop. With disk that negative pressure is what helps to fully disengage the brakes.
Or at least thats my understanding after too much reading on the subject.
I have read (but have not verified it) that gm started mounting the master higher on the fire wall and so that gravity would do the job.

 

[John Cunningham]

so leave the residual valve alone I think is what your saying?

 

[Crazydavez28]

Where did you get the parts for this? I have a leaky wheel seal on the back and would consider the disc swap if my drum setup is trashed. I pull a gooseneck occasionally and just turned up the trailer brakes to compensate for the crappy drums.

When I switch from my Tahoe to the K3500 I nearly hit people because the braking is so different.

 

[R422b]

If you had one i would remove it or put a disk master in it.
But keep in mind i am just going on research. I have verry little experience with drums.
Here is a link to wikipedia if you scroll down to system operation there is an old army training video that i found interesting.

https://en.m.wikipedia.org/wiki/Hydraulic_brake

It is slow but helped me a lot.

 

[Gibson]

John,, I'll take a shot at this, but I know that there are others on this forum that are more knowledgeable about the systems than I.
See, my conversion was done on a squarebody, and they didn't have any ABS system, and used a residual valve for the rear drums that was in the master cylinder,, they also didn't have that "lever linkage valve" that compensates for weight in the bed.
I think that in the '97s the residual for the rear drums might be part of the ABS system, but I don't know for sure.
But, if you used the calipers that use the large 2 13/32s pads, and if there is ~10 lbs residual pressure held in the rear brake line, then there will always be ~45 1/2 lbs of pressure on each pair of pads dragging on the rotors,, that's actually quite a bit,, it's split between two pads, so ~22.75 per pad,, (that pad size has ~4.5 sq. in. of surface area).
In the original front discs, there in no residual valve, the height of the master above the calipers in a stock vehicle is somewhere ~2ft,, and that provides a column height of fluid that has ~3/4ths of a psi at the front calipers, which works just fine.

 

[R422b]

So if you smell hot brakes after a long drive or your mileage drops you may have a problem.
Gibson i think you would have to multiply the presure by the caliper piston area in sq inches te get the force on each pad. Now how to convert that to ft lbs of drag is beyond me today.☺

 

[Gibson]

Gibson i think you would have to multiply the presure by the caliper piston area in sq inches

Yeah, I did.
A 2-13/32 dia piston has 4.547 sq in area,, times 10psi=45.47 lbs of force, then divide that by 2, the pressure is only on one pad, but the caliper slides to equalize the pressure between the both of them.
That 10lb residual is holding 10lbs in each rear caliper, so each one of the 4 pads is pressing on its rotor side with ~22.235 lbs of force.
Ideally you want very little residual on discs, they have no retractor springs, the only retracting force is a tiny bit of "spring-back" in the rubber boot, they always drag a little, and the column height of the fluid above the calipers is plenty of pressure to keep the pads in light contact.
With discs the difference between "relaxed" and "hard braking" is only a few thousands of an inch.

 

[R422b]

I guess i didnt read carefully enough.
Good post. ☺

 

[John Cunningham]

John,, I'll take a shot at this, but I know that there are others on this forum that are more knowledgeable about the systems than I.
See, my conversion was done on a squarebody, and they didn't have any ABS system, and used a residual valve for the rear drums that was in the master cylinder,, they also didn't have that "lever linkage valve" that compensates for weight in the bed.
I think that in the '97s the residual for the rear drums might be part of the ABS system, but I don't know for sure.
But, if you used the calipers that use the large 2 13/32s pads, and if there is ~10 lbs residual pressure held in the rear brake line, then there will always be ~45 1/2 lbs of pressure on each pair of pads dragging on the rotors,, that's actually quite a bit,, it's split between two pads, so ~22.75 per pad,, (that pad size has ~4.5 sq. in. of surface area).
In the original front discs, there in no residual valve, the height of the master above the calipers in a stock vehicle is somewhere ~2ft,, and that provides a column height of fluid that has ~3/4ths of a psi at the front calipers, which works just fine.

Thanks for taking a ME shot at it.......

Im not sure either where it might be but some research may show. As it stands now I have about 30 miles on the new brake system. I dont feel any drag from what I can tell anyway. I think 10 pounds I would feel and or smell if it was dragging. I am sure there must be a schematic of the system somewhere and I could always get rid of the abs but do not know how difficult that would be tied into the ecm etc. I have never felt the abs even work but no panic stops on ice or wet roads yet. I also would think that I could jack up the diff and run thru some gears and hit the brakes and release them and see if I could move the rear tires...I would think 10 pounds would prevent any movement. As it stands right now the system seems to be functioning normal but of course getting the correct info would be nice to know and correct if needed.
I am using a small JB6 single piston caliper and 12 inch rotor. I think we all know the rear does not do most of the stopping thats why I am upgrading to a larger 13 rotor and dual piston in the front.........but having said all of this the truck is remarkable in stopping smooth now with a light pedal. I also think these GMT 400's need less brakes than the newer pickups..my one ton K3500 looks like a baby next to the new one ton trucks yet it can tow a fiver just fine. Just was lacking in the brake dept. When this is all done I am going to attack the steering next. I cannot justify 60K or more for a new truck. All these young guys are driving new trucks jacked to the moon...I cannot imagine what their truck payments must be and for how long.