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4 wheel power disc conversion on 68 Cutlass master question
Hello,
I currently run a Wilwood front drum to disc kit. I am going to be installing a Right Stuff or Summit rear disc brake kit.
I know I need to upgrade my master and get a 4 wheel proportioning valve. I currently have the master for power drums with an 11 inch booster. The master does not have a prop valve, just 2 lines going directly from the master to under the body to the distribution block. There are tons of master and booster kits for this but I dont want to run new lines right now. What is the easiest way to upgrade without new lines? Are there any setups that will allow me to keep my current hardline location?
Also, would 1" or 1.25" master bore work best? Most kits for this conversion use a Corvette style master that hold even fluid on both sides.
I am also considering getting these 2 kits and using the existing 11" original booster to make things a bit easier.
Since I didn't get a reply, I ended up looking around and finding the answer myself. I called Wilwood and purchased their tandem master cylinder with their adjustable proportional valve. I will be able to do a direct swap on the master and add a 10lbs residual pressure valve to run drums on the rear in the meantime.
They recommended going with 1"-1.125" bore if the engine makes good vacuum with power booster. I got the 1.125" for my 11" factory booster. As explained to me, larger more will allow for a firmer pedal. For engines that do not make strong vacuum (Over 15psi) they recommend using a smaller bore.
Bore diameter has nothing to do with pedal "firmness". That is solely a function of properly bleeding the system and using flex lines that don't expand. The only thing bore diameter changes is the amount of pedal pressure required to achieve a given line pressure at the calipers. The smaller the bore, the less pedal pressure required for a given amount of braking force. Of course, a smaller piston moves less fluid per inch, so a smaller bore means longer pedal travel. Do not confuse either of these with pedal "firmness".
Bore diameter has nothing to do with pedal "firmness". That is solely a function of properly bleeding the system and using flex lines that don't expand. The only thing bore diameter changes is the amount of pedal pressure required to achieve a given line pressure at the calipers. The smaller the bore, the less pedal pressure required for a given amount of braking force. Of course, a smaller piston moves less fluid per inch, so a smaller bore means longer pedal travel. Do not confuse either of these with pedal "firmness".
I had a pretty good conversation with the Wilwood tech. Each bore size is going to have a different feeling because of how much fluid is being moved, how much vacuum pressure and how far the pedal travels. The result is either softer of firmer pedal. In my case using a 1.125" bore with an 11" booster is going to be more firm than soft (less pedal travel). He basically said what you just said.
Last edited by yeahbuddy; Jun 19, 2020 at 12:56 PM.
I had a pretty good conversation with the Wilwood tech. Each bore size is going to have a different feeling because of how much fluid is being moved, how much vacuum pressure and how far the pedal travels. The result is either softer of firmer pedal. In my case using a 1.125" bore with an 11" booster is going to be more firm than soft (less pedal travel). He basically said what you just said.
Sorry, but pedal travel is unrelated to firmness. A larger bore will require MORE force from your foot to achieve the same stopping distance as you would with a smaller pedal. If you consider this to be "firmness", you (and that tech) are mistaken. Requiring more force from your foot to stop safely isn't a feature. I use the smallest diameter M/C I can that still moves enough fluid to fully apply the brakes. I've read far too many posts where someone "upgraded" to four wheel disc, didn't properly match M/C and wheel cylinder diameters, and posted complaints of how the brakes are worse now than before. Get the correct engineering info.
Sorry, but pedal travel is unrelated to firmness. A larger bore will require MORE force from your foot to achieve the same stopping distance as you would with a smaller pedal. If you consider this to be "firmness", you (and that tech) are mistaken. Requiring more force from your foot to stop safely isn't a feature. I use the smallest diameter M/C I can that still moves enough fluid to fully apply the brakes. I've read far too many posts where someone "upgraded" to four wheel disc, didn't properly match M/C and wheel cylinder diameters, and posted complaints of how the brakes are worse now than before. Get the correct engineering info.
The Tech and OP are not mistaken in there terminology. They are using common terminology in the aftermarket performance and race car brake world to describe the affect of master cylinder selection on pedal feel. Its terminology used when you talk to the company engineers, not just the sales people. At least that's my experience when talking to Brembo, StopTech, and Wilwood engineers when seeking technical and setup advice on the phone as well as talking to pro motorsports teams and drivers at the track (a lot of teams in the lower IMSA and World Challege series are also in the business of selling parts, so they will tell you what works for a given chassis).
You also generally team the "firmness" discussion with a discussion of more or less progressive pedal when talking to a driver about how they prefer a car setup. Soft, spongy, and firmness terms are also used by drivers to describe pedal feel issues to pit crews and mechanics to help diagnose issue such as you described where there might be air in the lines, a leaking caliper seal or other fitting, or finding a bad wheel bearing that is causing excessive caliper knock back. I have changed master cylinders on numerous cars to suite the driver's preferences. Most drivers seem to prefer a larger master with a less progressive, higher, and firmer pedal. But, some, like the same car set up with a smaller master cylinder bore because they preferred a softer more progressive pedal.
I have had a number of on track instances in the past few years where my brake pedal became excessively soft at the end of a high speed straightaway. I had to pump the pedal to firm it back up while driving down the straight to prevent this and have any sort of ability to brake before going into the corner. While the pedal felt like there was excessive air in the lines and the system needed to be bleed, that was not the issue. After re-bleeding the brakes and checking for leaks, the issue persisted. In all of these instances it was a bad front wheel bearing resulting in pad knock back in the front calipers. Using your rational above, using the term "firmness" when talking to pit crew helping me diagnose and fix the issue would not have been correct because the root cause ultimately was not a improperly bleed system.
Another example, if you want to describe the "feel" of the below master cylinder kit once installed vs. the OEM factory setup, you would tell the driver that the setup will have a softer, more progressive pedal with longer pedal travel. Some driver's love this kit, other's hate how soft and long the pedal is once installed.
I realize you have an opinion concerning terminology, but in this case, I don't think its accurate or what is generally accepted. I completely agree that people often dive into brake kit swaps without analyzing the system as a whole, wind up with a poor performing systems, and the root cause is often a wrong master cylinder bore for the brake piston area.
The Tech and OP are not mistaken in there terminology. They are using common terminology in the aftermarket performance and race car brake world to describe the affect of master cylinder selection on pedal feel. Its terminology used when you talk to the company engineers, not just the sales people. At least that's my experience when talking to Brembo, StopTech, and Wilwood engineers when seeking technical and setup advice on the phone as well as talking to pro motorsports teams and drivers at the track (a lot of teams in the lower IMSA and World Challege series are also in the business of selling parts, so they will tell you what works for a given chassis).
You also generally team the "firmness" discussion with a discussion of more or less progressive pedal when talking to a driver about how they prefer a car setup. Soft, spongy, and firmness terms are also used by drivers to describe pedal feel issues to pit crews and mechanics to help diagnose issue such as you described where there might be air in the lines, a leaking caliper seal or other fitting, or finding a bad wheel bearing that is causing excessive caliper knock back. I have changed master cylinders on numerous cars to suite the driver's preferences. Most drivers seem to prefer a larger master with a less progressive, higher, and firmer pedal. But, some, like the same car set up with a smaller master cylinder bore because they preferred a softer more progressive pedal.
I have had a number of on track instances in the past few years where my brake pedal became excessively soft at the end of a high speed straightaway. I had to pump the pedal to firm it back up while driving down the straight to prevent this and have any sort of ability to brake before going into the corner. While the pedal felt like there was excessive air in the lines and the system needed to be bleed, that was not the issue. After re-bleeding the brakes and checking for leaks, the issue persisted. In all of these instances it was a bad front wheel bearing resulting in pad knock back in the front calipers. Using your rational above, using the term "firmness" when talking to pit crew helping me diagnose and fix the issue would not have been correct because the root cause ultimately was not a improperly bleed system.
Another example, if you want to describe the "feel" of the below master cylinder kit once installed vs. the OEM factory setup, you would tell the driver that the setup will have a softer, more progressive pedal with longer pedal travel. Some driver's love this kit, other's hate how soft and long the pedal is once installed.
I realize you have an opinion concerning terminology, but in this case, I don't think its accurate or what is generally accepted. I completely agree that people often dive into brake kit swaps without analyzing the system as a whole, wind up with a poor performing systems, and the root cause is often a wrong master cylinder bore for the brake piston area.
To me, a "firm" brake pedal is the opposite of a "spongy" brake pedal, which means there is air in the system. Again, the amount of force you have to generate with your foot to achieve a given brake force at the wheel is unrelated to the amount of pedal travel, only to the piston diameter and pedal lever arm. That's physics.
Of course, people also incorrectly call the passage in the block an "oil galley" instead of an "oil gallery"...
(and yeah, I realize that in the CSM, Olds uses both terms over the years)
