Rear Disk Brake Question

[455'ed80Regal]

I had to shim the backing plates to center the calipers on the rotors. I hope that helps with the issue you describe. Have not driven on them, yet.

I also had to shim my backing plates out. Went to my local Westlakes with dial caliper in hand and went through all the washers they had in my size and bought the ones that would get the closest, plus a little bit, to centering the rotor in caliper bracket. Then I filed them all down so all the washers were the same thickness at all four corners. A little anal I know, but that's me. I do have a set of the proper spacers, but they are too thick and will need to be machined down to the proper thickness. Just need to take them to work with me sometime and have it done.

 

[oldsmobile joe]

i finally did my conversion this past summer with ls1 rear discs and upgraded my fronts from c4hd to c5/blazer units. i did find it necessary to switch the master cylinder to a c3 corvette 1-1/8 bore unit. the original master gave the brake pedal a slow application feel, almost like a drum brake wasn't adjusted properly. i was lucky, no shimming required. no complaints of how the rears perform. i too kept the disc/drum proportioning valve. after trying several high speed stops in the rain, i found the rears would just start to lock up before the fronts, but not enough to cause a handling concern.

 

[pontiacgp]

The reason to change from a disk/drum proportional valve is due to the drum needs a 10 lbs residual valve to keep the shoes close to the drum. Lees than that the springs will pull the shoes too far off the drums. A disk brake only needs a 3 lbs residual valve and a 10 lbs residual valve should cause the pads to drag. I'm surprised you guys are not finding that issue with a disk/drum proportional valve

 

[oldsmobile joe]

actually, the 10lb residual valve everyone talks about is meant to keep the lips of the wheel cylinder cups pressed against the bore of the cylinder, which prevents air from entering the system during brake release. gm overcame the by installing a spring inside the cylinder.

the 10lb residual valve was not meant to keep the shoes close to the drum surface, the return springs on the shoes generate sufficient force to return the shoes to their hard stops. proper shoe adjustment and a properly operating star wheel and kicker will perform this service.

i too, was drawn into this fallacy. i spent many days researching factory service manuals,(1978,1980,1985 and 1986) reviewing proportioning valve diagrams and dissecting g-body valves looking for this info. now i can find this valve for other gm cars, and a lot of confusing misinformation, but no where can i find this valve on a g-body proportioning valve. cause if there is one, then in theory i should have a dragging brake issue. but i don't. even gm info is confusing, so i finally reviewed my tech notes i took back in 1983 and applying standard scientific methodology to my situation i can only conclude that there isn't one on our cars.

 

[pontiacgp]

the 10lb residual valve was not meant to keep the shoes close to the drum surface,

please try a 2 lbs residual valve for drum brakes and let me know how low the brake pedal is


"Ideal for drag racing, street rods, and many off-road applications, inline residual pressure valves from Wilwood retain a minimum brake line pressure to help eliminate excessive pedal travel. These 2 lb. valves are used in disc brake applications where the master cylinder is mounted below the horizontal plane of the calipers--in other words, where fluid drainback occurs, causing excessive brake drag. With drum brakes, 10 lb. valves are used to compensate for return spring tension in the drums. Wilwood residual pressure valves are made from billet aluminum and color-coded for easy identification.

 

[oldsmobile joe]

sorry, I'm unable to, do to my car is in storage and is inaccessible. i also don't feel a need to, your quote from Wilwood says it all.

These 2 lb. valves are used in disc brake applications where the master cylinder is mounted below the horizontal plane of the calipers--in other words, where fluid drainback occurs, causing excessive brake drag.

this is great advice for a vehicle like my 57 olds where the master is located under the floor. but my master is located on the firewall, well above the horizontal plane of the calipers.
now I'm not closing the door on this subject, i believe open discussions like this are very informative, educational. and I'm humble enough to understand I'm not perfect or knowledgeable in all matters. but i will need to see proof of these valves in use from the factory in a similar application for to me to be swayed into using them.

 

[pontiacgp]

sorry, I'm unable to, do to my car is in storage and is inaccessible. i also don't feel a need to, your quote from Wilwood says it all.
this is great advice for a vehicle like my 57 olds where the master is located under the floor. but my master is located on the firewall, well above the horizontal plane of the calipers.
now I'm not closing the door on this subject, i believe open discussions like this are very informative, educational. and I'm humble enough to understand I'm not perfect or knowledgeable in all matters. but i will need to see proof of these valves in use from the factory in a similar application for to me to be swayed into using them.

I was responding to your post about drum brakes wherein you posted "the 10lb residual valve was not meant to keep the shoes close to the drum" and yes Wilwood says it all..."10 lb. valves are used to compensate for return spring tension in the drums"

 

[oldsmobile joe]

Ok

 

[axisg]

If that's the case then I should be noticing a build up of brake dust on the rear wheels ( from the pads dragging ) which I do not.

 

[marcar1993]

Same here, I have the factory setup on my car now, and the 8.8 ford rear in my car at the moment has disc brakes. There is no drag, and they do not hang up. With the rear in the air, the wheels spin freely just like a front wheel.