You have a 78, which if you have the stock booster, the Impala SS, LHD S10, or later caprice master will NOT fit. There is a pilot on the booster that the master slides into that increased in size in 82 or 83. You can't put a 83+ aluminum body master in a car that had a cast iron master.
I put a 90 caprice wagon master on my 86 2+2 that helped when I did the disk conversion. I agree I had the same issue as you. Very poor rear braking with the stock master. I swapped the proportioning valve first and it did nothing. I read into the fact that drum masters have check balls installed to keep some residual pressure in the rear and drilled that out and still had bad rear brakes. I installed that larger bore B body master and it helped.
Raybestos MC39079 is for 79-81 rivs and toros with disk/disk setups. I had it on my 80 Century and it fit. 1" bore. The disk/drum cars have the same master as a typical G body so you need to get the disk/disk for the larger bore. 77-81 WS6 trans ams have a large disk/disk bore master also but they have a cast iron revivor that I don't really like.
Looks like the Raybestos is out of stock on Rock auto but they have these options too.
Thanks 81Cutlass. I crossed referenced the trans am masters that are listed above, and the AC Delco master is the same part number as the Riviera master that i just ordered. I actually put the raybestos in my cart but it kicked me out because it wasn't in stock. I'm starting to feel confident now that I'm getting the right part. I think I will be able to start pi**ing off my neighbors with this car pretty soon.
I have to thank everyone that helped me out with this project. I was pulling my hair out, and I don't have that much to start with. I got the Riv MC from RockAuto at 11am, and I had it bled, put on, and the lines bled by noon. With working rear calipers!!!!!! So many of these posts I've noticed don't always have closure, but I'm here to say that the 80 Riviera Master (rear disc model) is a solution for the disc brake conversion from Quick Performance. The pedal is not as hard as my daily driver Malibu, but my DD Malibu is 35 years newer than my project car Malibu. Thanks again everyone. You guys are the greatest!
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