i converted my 85 Monte to rear disc. I'm to the point where I'm changing the master cylinder and one that I'm looking at is off a 79 corvette with the disc/disc reservoir. Anyone done this?
Brake question
- Thread starter timmyadkins
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The M/C doesn't need to be changed. Some suggest the proportioning valve does though.
https://gbodyforum.com/threads/disk-disk-proportioning-valve.57215/
https://gbodyforum.com/threads/disk-disk-proportioning-valve.57215/
The G body master cylinder is fine...no need to change it. The vacuum booster is another story. Many G bodies have a small diameter single stage design. You can bolt on a dual diaphram and larger diameter B body booster for more assist. The drum disk proportioning valve should be changed unless you like your rear end coming around under hard braking. I used a 79' Cadillac Seville proportioning valve for disk/disk as a direct replacement and it worked great.
I would just comment, I never changed my proportioning valve and my car stops normally. No rear brake lock up. I'm not saying one never needs to change their proportioning valve, but certainly if you are experiencing rear brake lock up, the proportioning valve may very well be the culprit. 🙂
drum brakes need a 10 lbs resistance to keep the springs from retracting the shoes too far from the drum, disk brakes only need a 3 lbs resistance to keep the pads close to the disk
here is my experience with disc brake upgrades I've made over the years.
i started with converting my fronts to '96 corvette hd rotors and calipers. no other changes. rear brakes would lock up prematurely.
many years later, upgraded rears to ls1 camaro rear disc rotors and calipers. no other changes made. rear brakes worked properly, no premature lock up during panic stops, but i didn't drive this very hard before i performed the next mod.
upgraded front brakes to 2000 corvette 12.8" rotors and calipers. this caused me the most frustration. pedal feel was weak, long pedal travel, like i had air in the system. i would run out of pedal reserve. not a happy feel.
switched master cylinder from factory 79 cutlass to a master for an '87 monte. pedal feel got better but still didn't feel right. switched master to a mid '70s corvette master, sorry i don't remember if its a 1" or 1-1/16" bore. pedal feel is now normal.
as far as the proportioning valve is concerned. i have not changed it. i have safely tested the fore/aft balance for street driving, no premature lock up was experienced, wet or dry.
also, these valves do not have a residual pressure valve inside them. contrary to what you might read, they don't. i can not find any factory diagrams (gm, kelsey-hayes, bendix) indicating such for a valve in the gbody. as further evidence, if you look up rear wheel cylinder o-haul kits, they show a spring and cup assembly to keep the rubber cup lips in contact with the wheel cylinder bore.
i have been quite satisfied with this arrangement. 3000plus miles including 1000 miles towing another cutlass from alabama to minnesota. oh yea, and this was in the rain. think of a 3000lb trailer with no trailer brakes. my brakes performed flawlessly.
***results may vary***
joe
i started with converting my fronts to '96 corvette hd rotors and calipers. no other changes. rear brakes would lock up prematurely.
many years later, upgraded rears to ls1 camaro rear disc rotors and calipers. no other changes made. rear brakes worked properly, no premature lock up during panic stops, but i didn't drive this very hard before i performed the next mod.
upgraded front brakes to 2000 corvette 12.8" rotors and calipers. this caused me the most frustration. pedal feel was weak, long pedal travel, like i had air in the system. i would run out of pedal reserve. not a happy feel.
switched master cylinder from factory 79 cutlass to a master for an '87 monte. pedal feel got better but still didn't feel right. switched master to a mid '70s corvette master, sorry i don't remember if its a 1" or 1-1/16" bore. pedal feel is now normal.
as far as the proportioning valve is concerned. i have not changed it. i have safely tested the fore/aft balance for street driving, no premature lock up was experienced, wet or dry.
also, these valves do not have a residual pressure valve inside them. contrary to what you might read, they don't. i can not find any factory diagrams (gm, kelsey-hayes, bendix) indicating such for a valve in the gbody. as further evidence, if you look up rear wheel cylinder o-haul kits, they show a spring and cup assembly to keep the rubber cup lips in contact with the wheel cylinder bore.
i have been quite satisfied with this arrangement. 3000plus miles including 1000 miles towing another cutlass from alabama to minnesota. oh yea, and this was in the rain. think of a 3000lb trailer with no trailer brakes. my brakes performed flawlessly.
***results may vary***
joe
you are correct...
"How does a disc brake master differ from a drum brake master?
A drum brake master will differ from a disc brake master in two ways. The amount of fluid that a drum brake master has to move is less than disc brake. Drum brake masters have 10 lb residual valves at the outlet to keep a residual pressure on the drums. If you use a drum brake master for disc brakes you would move an insufficient volume of fluid and the disc brakes would drag because of the residual valves."
http://www.mpbrakes.com/master-cylinders-faq
yes this is correct.
i was taught this in tech school, but this usually pertains to masters mounted at a level even with or below the wheel cylinders, like '57 olds that is mounted under the floor, not in masters that are mounted high on the firewall like our gbodies. i can find no factory diagrams indicating this is on our gbody master cylinders. between the weight of the fluid and the internal spring/cup assembly of the wheel cylinders, it would be superfluous.
joe
sorry, it was "my word of the day" i had to find someplace to use it. lol
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