HELP Disc Brake Conversion Help Please

Rgpmalibu

Not-quite-so-new-guy
Oct 7, 2020
24
13
I have a 1978 Malibu. I swapped the rear end for a 9" from Quick Performance. I used the calipers they gave me without parking brake. I have the stock brake booster original to the car. I did switch the brake proportioning valve to a disc/disc valve. The master cylinder is the stock one (disc/drum), but it is brand new. I have bench bled the master, twice (the most recently with the syringe method) and bled the brakes with a Motive Power bleeder three times at 15 psi. I feel like I have some pedal until I start the car. When I start it up, the pedal goes to the floor. When I put the car in reverse, on the jack stands, the rear wheels won't stop turning when I apply the brakes. I really don't feel like I have air in the lines - I've been pretty thorough. I am a researcher, and I have heavily scoured all the posts I could possibly find, with inconclusive results. The conclusions that I have drawn is that either the master is not pushing enough fluid to make the rear calipers clamp down, or the master pushrod is too short, but I think that is a longshot, because everything I'm using (booster and M/C) are parts specific to the model year. I am wondering if anyone else has had this problem, and if they've had any success. I just bought a new master from an 85 Eldorado hoping that the disc/disc master might solve the problem, but it didn't fit the booster. I'm feeling that I'm using the wrong master for the system, but I need to maybe find a master that would fit my booster, as it seems to be working properly. Any help or suggestions would be greatly appreciated!! I've been putting this thing together for a year, and I am so close, but I'm afraid to drive the thing with no rear brakes.
 

malibudave

Greasemonkey
Mar 12, 2010
162
43
Houston, TX
You need a larger bore master cylinder. The master cylinder you have is 24mm or a roughly 15/16. It is too small for your front and rear brake setup.

Also, the reservoir for the rear brakes is quite small and may not hold enough fluid in reserve for the rear calipers.

I would suggest an 1.125" bore master cylinder.

Which master cylinder will work with the angled firewall may be a little harder.

I know a 2000 Express Van has a 1.125" bore master cylinder, but the reservoir is not angled, but it should bolt up to your vacuum booster.

You could also do the B-body vacuum booster and master cylinder swap.


This swap may be better as the master cylinder has an angled reservoir. The master cylinder will most likely not work with your 78 vacuum booster as the housing below the flange is most likely to large to bolt onto your stock booster. A newer 1982 and up Gbody vacuum booster should work with the b-body master cylinder.

Let me know if you have any additional questions.
 

Rgpmalibu

Not-quite-so-new-guy
Thread starter
Oct 7, 2020
24
13
It looks as though, from the pictures mind you, the express van master has the smaller flange that goes into the booster that I have. The fitting sizes are the same too. How concerning is it that the master isn't angled. Could I run this master at an angle? Is this something that's done, or is it something I shouldn't even try? I would rather not have to replace the booster too, for many reasons, not the least of which is that it looks like a PIA. I'm 6'7" - it's not super easy for me to work under the dash. Plus my booster seems to be working just fine. What do you think?
 

malibudave

Greasemonkey
Mar 12, 2010
162
43
Houston, TX
The reservoir shouldn't be an issue, but it may look funny.

Just make sure that the front reservoir is not overfull as it may syphon to the rear reservoir and overflow over the back of the reservoir.
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
2,044
113
Upstate NY
Dave's recommendation will work. I went a little different route than the B-body program because, like you, I didn't want to replace the booster. The issue with the booster is that in the '80 and earlier cars the booster can only accept a smaller bore master cylinder. I used an '80 Riviera master cylinder - it fits the small bore booster and has increased capacity for larger fronts and rear discs. It was cheap and easy, and it's smaller dimensioally than the B-body stuff.

I've documented the part number here somewhere lol. But I went from the stock master to the Rivi master and raised the pedal back to original feel. I will point out though, I had C6 brakes installed on the front. But it should work fine for the issue that you're experiencing if you don't want to replace the booster. Lastly, the proportioning vave swap was almost definitely unneeded. When I changed from the stock prop valve to an aftermarket disc/disc valve it had almost no effect on braking.

God luck - Jim
 

Rgpmalibu

Not-quite-so-new-guy
Thread starter
Oct 7, 2020
24
13
Malibudave and 64nailhead, I really appreciate you guys taking the time out to give really good answers to yet another part of my build that I have been left scratching my head about. I gotta tell you though, I'm loving every minute of this. I should have bought a beat up Malibu years ago! Oh, and Malibudave, that funny looking angled master wouldn't interrupt the theme of my car at all. It's pretty funny looking to begin with. Thanks again guys!!
 

Streetbu

Know it all, that doesn't
Supporting Member
May 22, 2011
2,146
113
Central NY
Just make sure your bleeders are on top of the caliper, and not underneath. Seen many people accidentally put them on the wrong side and then wonder you can't get all the air out. That being said, you would still need the larger master as stated above, but it would make the pedal much better for now.
 
Last edited:

Rgpmalibu

Not-quite-so-new-guy
Thread starter
Oct 7, 2020
24
13
I see what you are saying Dave. Put in the Express van master cylinder, but put the caprice reservoir on top so that it is angled.

Jim, do you have a part number for that Riviera master. Does that have a 1 1/8' bore as well? I looked at RockAuto, and the ones I saw have a 24 mm bore. What makes them work better?

Streetbu, bleeders are on top. I even unbolted the caliper to make sure that the bleeder was at the very top when I bled them.
 
  • Like
Reactions: Streetbu
G-Body Performance Upgrades

GBodyForum is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com. Amazon, the Amazon logo, AmazonSupply, and the AmazonSupply logo are trademarks of Amazon.com, Inc. or its affiliates.

Please support GBodyForum Sponsors

Classic Truck ConsolesDixie Restoration DepotMike's MontesP-S-TSouthside Machine PerformanceUMI Performance

contact[email protected]for info on becoming a sponsor