Hold on now. Switch to hydroboost brake assist from vacuum brake assist should not be thought to require a new master cylinder. The point of this thread is to discuss the retrofit of the "assist" part of the braking system. I would hesitate to advise someone to change the stock master cylinder (which has been designed by Ford based on the front caliper size and rear wheel cylinder size) without also advising to change out the front and rear brake calipers/cylinders to match the displacement of the new master cylinder. Changing the master cylinder without changing the associated "downstream hardware" could result in either a too hard or too soft pedal or too long or too short pedal travel, based on which way you were to size it.
You hold on! You are forgetting that moving the pushrod pin higher up on the pedal changes its leverage. For instance assuming 2" travel down at the pedal pad (completely random number used for illustration purposes only), a pedal with its pushrod pin closer to the pedal pivot will provide the master with shorter stroke but with higher force as compared to a pedal with its pin further away from the pivot. Thus while the large master pushes out more fluid volume per length of stroke the shorter stroke provided by the pedal will partially make up for it. At the same time the loss of hydraulic advantage due to the larger bore is made up for by the increased force applied by the pushrod. In simple terms:
a) if you keep your vacuum master but switch to hydro pedal you will end up with softer pedal with more travel to it
b) if you switch to hydro master but keep the vacuum pedal you will end up with hard pedal with very little travel
c) if you switch both master and pedal together the negative effects of each of them will mostly cancel one another
Also, for OBS trucks, it is a fact that the F-Super Duty master cylinder does not have a provision for the cruise control brake pressure switch. How do you switch and keep your cruise?
Fact you say? In whose personal reality? Surely not in mine, or in these folks' for that matter:
http://www.autozone.com/autozone/pa...43aZ8kn47?itemIdentifier=532395_429915_0_5520
Again, the point of the hydroboost swap is not to alter the hydraulic design of the closed brake circuit (thats a whole other can of worms), it is to simply replace (upgrade depending on who you ask) the power brake assist function.
Which would be true if the dang Ford engineers did their job right and made the hydro line up with the vacuum pedal. You know, just like how GM did. For some 30 years in a row! But no, someone decided to be a *****. So we end up with this dang mess.
So the rest of the braking hardware (calipers/wheel cylinders) has no affect on sizing of the master cylinder then? Have to remember that while the F-Super Duty has a larger bore master cylinder, it also has larger brakes for increased GVWR. You size the brakes for the GVWR, then size the master to provide the required volume to the brakes, then design the power assist system to provide the proper pressure into the master cylinder.
Sure it has an effect. That's why people who use the full hydro swap (master + booster + pedal) report having somewhat reduced pedal travel, cause even with its matched pedal the master still pushes out slightly more fluid than the calipers and wheel cylinders are designed for. Yes that leads to the loss of some hydro-mechanical advantage. But since the hydrobooster dang near doubles the assist force applied to the master, the overall feel for the pedal is better.
All the anecdotal stories posted by folks post-hydroboost swap about how the brakes are so much better is misleading. The brakes aren't any better than before (unless you modified the brakes). Its only easier to impart more force into the hydraulic brake system (via the master cylinder) with the hydroboost setup; result, you don't have to push on the pedal as hard as before to achieve the same braking force at the wheels.
That actually goes both ways. Lemme ask you this, do you know what pedal force you're capable of? I know mine around 175 lbs measured at the pad. In a panic stop I'm gonna put that force down regardless of what booster I have. And if truck is lightly loaded either booster will cause the wheels to lock up (and RABS to kick in, if present and active). But when you're loaded heavy it takes lots more clamping force at the wheel brakes to cause the same lockup. That force is caused by line pressure, so if your line pressure is higher the clamping force will also be higher. That's the real advantage of the hydroboost system, it allows you to turn the same pedal force you always apply into higher line pressure and thus higher clamping force at the wheel brakes. Which really is only good if you have enough traction to use it. But if you're loaded so heavy traction is guaranteed either way, then hydroboost will indeed stop the truck faster cause of the higher clamping force it's capable of.
Yep, just like this guy did. Doesn't seem to difficult. Allows the use of an off the shelf hydroboost unit (no mounting plate drilling or firewall butchering required) and achieves the proper operating angle and leverage on the pushrod into the hydroboost unit.
Wait, did I see you mention pedal leverage? So you were aware of its effects, yet you conveniently left it out of the discussion about effects of master size. Why is that? Do you not realize the hydrobooster really doesn't care about the leverage that's pushing on it, and that said leverage is there only cause of the master size? Consider this - myself and at least one more person here are running factory vacuum pedals and masters with hydroboost between them - and both trucks stop ridiculously good with that setup. The catch is we don't use Ford hydroboosters - our boosters of choice require spacing out about 2" from the firewall, and the resulting adapters allowed us to line up the boosters to the vacuum pedal pin. Which is the situation you were describing at first - factory hydraulic circuits, factory mechanical linkage to actuate them, just stronger assist between the two. And it's the same thing that will happen of someone simply moves a Ford hydrobooster down 3/4" to line it up with their vacuum pedal and then reuse their vacuum master as well. But the moment you swap in the hydro pedal, even if it's only so the Ford hydrobooster don't bind, you will need to increase the size of the master to compensate for the reduced stroke of the pedal. Because like you said the wheel brakes require a fixed amount of fluid, and said amount of fluid depends on BOTH the bore AND the stroke of the master.
