Upgrade from single master to vacuum boost dual master - 63 C20

[theastronaut]

Originally Posted by Lugnutz65

The Astronaut,
Let’s see if you agree.
A smaller bore MC is less touchy because it pushes less fluid during the stroke compared to a larger bore making the same length stroke. Therefore a smaller bore MC is less sensitive or “touchy” than a larger bore MC.

Another method that can reduce the “touchy” feeling of boosted disc brakes is to drill a new hole where the pushrod connects to brake pedal arm. Most articles suggest moving the connection hole down 1”. Moving the pushrod down reduces the mechanical leverage from 9:1 to maybe 7.5 to 1.

Yes, there are multiple ways to tune pedal effort and pedal feel.


Pedal ratio- stroke of the pedal compared to stroke of the master cylinder.

Bore size of the master cylinder.

Bore size of the caliper/wheel cylinder.

Booster diaphragm diameter.

Engine vacuum affects the effectiveness of a booster.

Pad/shoe compound.



Just going to a smaller bore will make more line pressure at the same pedal effort, but you'll need more pedal travel to move the same amount of fluid. I like that because just like effort, you can more accurately hold at the maximum amount of pedal effort if the pedal travel is longer. You develop muscle memory of how far to press the pedal along with how much pressure to push with to make the brakes lock up, so I like more travel to make it easier to develop muscle memory. I like to repeatedly practice panic stopping to learn how the vehicle reacts to varying pedal effort, and longer travel and higher effort makes that easier to learn.

A booster throws an extra variable in that isn't always needed, and while changing the pedal ratio can help add some pedal effort and travel back you're somewhat defeating the purpose of adding a booster at that point. For me I'd rather skip the booster and tune the pedal feel with MC bore size once the caliper bore size is settled and pad material is selected.


A lot of this is forgotten and looked over in the era of ABS that does all of the work for you so the brakes don't lock up... just stomp the pedal and it does the thinking for you. But old trucks don't have ABS, so having brakes that can be applied effectively and repeatably and intuitively is important. Slapping on a booster without tuning the system can be a very bad thing. A boosted system that has been tuned can be great too. It comes down to personal preference, but most people don't get that involved because they never plan on pushing the truck to it's performance limit, but because of that they have no idea how it'll respond in an emergency braking situation. That's not safe when you don't have ABS to think for you.


The amount of tire grip can play a part in this too. A sticky racing slick will take more pedal effort to lock up than a common every day all season tire using the same braking system. You need enough clamping force at the wheel to lock up the tire you're using at a reasonable pedal effort. If you can barely lock up all season tires using all your effort, the truck will never stop shorter if you change to a grippier tire because it can't take advantage of the extra grip- you'd need more mechanical advantage to make more line pressure to take advantage of the extra grip.

This goes the same with speed- the faster you go you'll need more line pressure to lock up the same tire. A tire that's easy to lock up at 30 mph takes a lot more effort to lock up at 80 mph. If you're regularly braking hard at 100+ mph, like on a road course, then you might need the added assistance of a booster to make enough line pressure to stop as short as possible at high speeds. In that case a manual system could work but if you're skinny like me your leg might get tired of the higher pedal effort needed after a few laps.