Rear Disk conversion trouble

[malibudave]

On later model GM cars (70s and up) The 10lb residual valve for rear drum brakes will be in the prop valve and NOT in the master cylinder.

The 10lb residual valve is needed for rear drum brakes to keep the springs from pulling the shoes to far away from the drums. Keeping a disc/drum prop valve my cause the rear shoes to drag against the rotor because of the 10lb residual valve.

Make sure the bleeder screw is at the top of the rear caliper.

Your stock master cylinder should be fine with a disc/disc setup, but your pedal stroke may be slightly longer because calipers require more volume of fluid to fill than a wheel cylinder.

With those calipers, the pads are adjusted for wear by engaging the parking brake mechanism.

When bleeding the master cylinder it is best to use the plug method.
1. Plug both outlets of the master cylinder (rear port is 1/2-20, the front port is 9/16-18).
2. Fill the reservoir the proper amount of fluid.
3. Cycle the piston 3 to 4 times with a blunt rod.
4. After cycling the piston 3 to 4 times, the pedal should be rock hard.
5. If the pedal does not become rock hard, there is most likely something wrong with the master cylinder.
6. If the pedal does become rock hard, hold the piston in for 60 seconds.
7. If the piston starts to sink, then there is most likely something wrong with the master cylinder.
8. If the piston holds pressure and doesn't sink down the bore, the master cylinder is good and you know there is NO air in the master cylinder and it hold pressure.
9. Install the master cylinder with the plugs and bleed the rest of your system.

If you have other brake issues, using the plug method to bleed your master cylinder, you will know it is NOT your master cylinder giving you the issues.

 

[jakes87SS]

if your using th old master they will get a build up in the piston where they normally stop, then a few bleeds of the piston going over that spot can tear the seal on the piston and cause the brakes to be terrible, that or the new prop valve has air stuck in it, but the master is most likely the cause it happened to me when i did my blazer front and disc rear upgrade and bled them out.

 

[64nailhead]

Honestly not sure what kind of caliper they are. But the bleeder is on the bottom on the rears. The stock front ones are on top though.

There's your sign - you won't get the air out of those calipers via normal bleeding due to the air rises to the top of the chamber, but the bleeder is on the bottom 🙁 . You can unhook the hose and push fluid through the bleeder, and then bleed normally through the line. It's somewhat messy, but it will work. Otherwise you have to remount the calipers correctly.

Also, the proportioning valve will need to be changed to disc/disc.

 

[oldsmobile joe]

you found the problem. swap the calipers to the proper sides and bleed them.
there is no residual pres valve on the gbodies. not in the master or in the proportioning valve. i've looked for it in the service manuals, non exist. older cars have them, gbodies don't. please google for reference.
i currently have c5 vette front disc brakes and ls1 rear disc brakes on my brown calais. still have the original proportioning valve. i did swap out the master cylinder after chasing the spongy pedal issue. the stock master will not move enough fluid for the rear discs.
i have about 12k miles on my set up, and it performs flawlessly.

 

[79longroof]

Yeah I don't know why it never occurred to me that those breeders weren't going to work right on the bottom. Once the first person mentioned it, the light bulb in my dull head went off.
I immediately went out, spent an hour switching the calipers to the right side. Bleeding went as expected after that.
Did end up replacing the master cylinder because my old one began leaking out the rear seal while bleeding. Got it bled, much better now. Still not perfect however. Definitely stops better than before but still a little too much pedal travel. I guess it still has air in it somewhere that still needs bled.

 

[pontiacgp]

Yeah I don't know why it never occurred to me that those breeders weren't going to work right on the bottom. Once the first person mentioned it, the light bulb in my dull head went off.
I immediately went out, spent an hour switching the calipers to the right side. Bleeding went as expected after that.
Did end up replacing the master cylinder because my old one began leaking out the rear seal while bleeding. Got it bled, much better now. Still not perfect however. Definitely stops better than before but still a little too much pedal travel. I guess it still has air in it somewhere that still needs bled.

how far off are the rear pads backing off from the rotors?

 

[fleming442]

You have to adjust them with the e-brake, then bleed the hell out of them.

 

[64nailhead]

Where did you get the disc setup/kit? And will it fit inside a 15" wheel?

 

[Bonnewagon]

On later model GM cars (70s and up) The 10lb residual valve for rear drum brakes will be in the prop valve and NOT in the master cylinder.

DOH! That is correct. I forgot about the combination valve. The TA disc/disc valve is also easy to get and use. But a large bore master is still a good idea for these cars. My old Malibu wagon had a tiny bore which gave very easy pedal effort while causing way too much pedal travel. I prefer more effort with less travel like a 1-1/8" bore gives.

 

[79longroof]

Yeah I don't know why it never occurred to me that those breeders weren't going to work right on the bottom. Once the first person mentioned it, the light bulb in my dull head went off.
I immediately went out, spent an hour switching the calipers to the right side. Bleeding went as expected after that.
Did end up replacing the master cylinder because my old one began leaking out the rear seal while bleeding. Got it bled, much better now. Still not perfect however. Definitely stops better than before but still a little too much pedal travel. I guess it still has air in it somewhere that still needs bled.

Where did you get the disc setup/kit? And will it fit inside a 15" wheel?

The kit came with my QP rear. Have no idea what "brand" is. I'm guessing it's a Chinese copy of something. And I have my stock wheels on it. Had to buy some basic wheel spacers to get them to clear the rotors. Sorry I don't remember how thick they are. But you can get them from the parts store.