73-87 Rear Disc Brake Conversion Problem

[GreenRacer 64]

Hi All,
I have a '78 Stepside and i bought a Cadillac Eldorado rear brake conversion kit to upgrade my truck. Unfortunately, I cannot get the brakes to firm up after bleeding. I understand they are notoriously hard to bleed and I either still have air in them or i've got a missmatch of hardware.
This is the hardware I am running:
*JEGS Rear Disk Brake Conversion 965-TRD4745
*JEGS Disc/Disc Proportioning Valve 555-631292
*The Right Stuff DBMC05 Disc/Disc Master Cylinder (1" bore)
*Brake lines are factory. 1/8" front, 1/4" rear
*Front brakes are stock calipers
*All new soft brake lines

I changed out the out the original Disc/Drum master cylider and proportioning valve but I cant get the brake pedal to firm up after many, many attempts to get them bled.

When it comes to bleeding I've tried gravity, brake pedal pumping, and a vacuum bleed system that I purchased. I've always positioned the bleed valve at the highest spot and even gone so far as to tap the rears with a hammer to knock loose any air if it was trapped in the rears. I've even taken the rears loose while vacuum bleeding, tapping them with a hammer and roating them to try and remove any trapped air. Finally, I have an ASE Certified Mechanic best buddy come over and we rebled the entire system starting at the master cylinder we bled each component only to continue to have a soft pedal.

If I pinch off the rear center bake line so only the front brakes are involved, the pedal is firm. This confirms the problem is in the rear.

I could really use some help. Do I have the right Master Cylider and Proportioning Valve? I find it hard to believe I still have air in the system. I've bled them no less than 20 times and i've been bleeding brakes for 20 years. Thanks for your feedback!

 

[AuroraGirl]

Hi All,
I have a '78 Stepside and i bought a Cadillac Eldorado rear brake conversion kit to upgrade my truck. Unfortunately, I cannot get the brakes to firm up after bleeding. I understand they are notoriously hard to bleed and I either still have air in them or i've got a missmatch of hardware.
This is the hardware I am running:
*JEGS Rear Disk Brake Conversion 965-TRD4745
*JEGS Disc/Disc Proportioning Valve 555-631292
*The Right Stuff DBMC05 Disc/Disc Master Cylinder (1" bore)
*Brake lines are factory. 1/8" front, 1/4" rear
*Front brakes are stock calipers
*All new soft brake lines

I changed out the out the original Disc/Drum master cylider and proportioning valve but I cant get the brake pedal to firm up after many, many attempts to get them bled.

When it comes to bleeding I've tried gravity, brake pedal pumping, and a vacuum bleed system that I purchased. I've always positioned the bleed valve at the highest spot and even gone so far as to tap the rears with a hammer to knock loose any air if it was trapped in the rears. I've even taken the rears loose while vacuum bleeding, tapping them with a hammer and roating them to try and remove any trapped air. Finally, I have an ASE Certified Mechanic best buddy come over and we rebled the entire system starting at the master cylinder we bled each component only to continue to have a soft pedal.

If I pinch off the rear center bake line so only the front brakes are involved, the pedal is firm. This confirms the problem is in the rear.

I could really use some help. Do I have the right Master Cylider and Proportioning Valve? I find it hard to believe I still have air in the system. I've bled them no less than 20 times and i've been bleeding brakes for 20 years. Thanks for your feedback!

might not be the right volumes being used
Maybe a brake line diameter or bad piston seal(s) situation
The front calipers are still the original non-low drag brakes right? Hows your pushrod lengths

 

[bucket]

The Eldorado calipers have a bad habit of not staying adjusted. So when you hit the pedal, it's soft until the pad reaches the rotor. Do you have the parking brake hooked up and functioning?

 

[Edelbrock]

I'm not sure that pinching off the rear line will isolate the problem. For example, if you were to take one of the brake lines out of the master cylinder and plug that hole with a bolt, then the brake peddle would be firm as well, since the piston inside the master cylinder can not compress fluid.

As far as bleeding, I worked on a rig once and I bled and bled and just could not get a firm peddle. I decided to give up and drive the vehicle to a shop and pay them to try. 2 seconds into driving it I realized I was chasing a ghost. The brake worked GREAT, and although the peddle could be pressed down farther than I was expecting before it became firm, the brakes would all lock up long before that. So brakes lock at 3 inches, but if I pushed hard I could get the peddle to go down 6 inches. I recommend driving it and see how it responds. After all, the whole idea is to get good brake performance, so if they perform really well - then there is no more work to be done. Fixed is fixed.

 

[Grit dog]

^Yet that still meant something wasn’t working as it should, even though the vehicle had good braking power. That was a coincidence that the issue didn’t actually result in poor braking power, as you described it.
Idk why the OP is having that issue definitively, considering he appears to have bled them more than enough.
Although I suspect it may be something with the aftermarket master cylinder and pushrod length or stroke. But have never done a rear disk conversion on these trucks (or any vehicle).
It’s hard to beat a good working set of rear drum brakes especially at the cost and effort to retrofit. Unless maybe for a racing application or something.

 

[bucket]

I'm just going to emphasize, the Eldorado calipers often cause problems. They cause problems on Eldorados and they often cause problems in rear disc brake conversions. They are a very crappy caliper.

 

[legopnuematic]

I'm just going to emphasize, the Eldorado calipers often cause problems. They cause problems on Eldorados and they often cause problems in rear disc brake conversions. They are a very crappy caliper.

And that at this point they have been remanufactured who knows how many times. Last I knew there were no new ones being made, all rebuilt/reman.

 

[Ricko1966]

What is the piston diameter in your stock calipers and your eldorado calipers. Was your master designed for a 4 wheel disc. Or 2 wheel disc 2 wheel drum. Did you adjust your calipers and cycle your ebrake levers?

 

[AuroraGirl]

the caliper also needs to be positioned so that the outboard pad only has a little ways to be "pulled" into the rotor, and that the inboard pad has to be basically dragging as close as it can. if its not like that, it will need to extend the piston further to start braking with any actual effect (dropping pedal)

 

[AuroraGirl]

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[Bennyt]

What is the piston diameter in your stock calipers and your eldorado calipers. Was your master designed for a 4 wheel disc. Or 2 wheel disc 2 wheel drum. Did you adjust your calipers and cycle your ebrake levers?

This! Ratcheting the p-brake is a must

 

[Edelbrock]

Sounds like a dumb question, but I have seen this happen.

The way you have them installed, are the bleeders on the top or the bottom of the caliper? Some rigs can actually allow the caliper to bolt on upside down.

 

[cadillac_al]

That's a pretty interesting procedure for adjusting the park brake. I have had my Cadillac for 30 years and worked on the rear brakes a few times over the years but I don't recall reading about adjusting the park brake arm in the service manual. My rear brakes have always felt more than adequate and I sometimes do a lock up test on my dirt driveway but the park brake has always been very weak. I always assumed that was just the nature of the Cadillac rear disc brake system. I am almost curious enough to try this procedure on my Caddy

 

[AuroraGirl]

Sounds like a dumb question, but I have seen this happen.

The way you have them installed, are the bleeders on the top or the bottom of the caliper? Some rigs can actually allow the caliper to bolt on upside down.

gmt900?

 

[bucket]

That's a pretty interesting procedure for adjusting the park brake. I have had my Cadillac for 30 years and worked on the rear brakes a few times over the years but I don't recall reading about adjusting the park brake arm in the service manual. My rear brakes have always felt more than adequate and I sometimes do a lock up test on my dirt driveway but the park brake has always been very weak. I always assumed that was just the nature of the Cadillac rear disc brake system. I am almost curious enough to try this procedure on my Caddy

I haven't seen the procedure laid out like that before either. I'd assume that they want you to do it that way, just because it seems more professional than manually ratcheting the lever a whole bunch of times. But that's just my assumption.