Note that these little aftermarket vacuum boosters do not provide much amplification, especially the single diaphragm versions. Back in the early days when I owned / operated Unlimited Racing in Detroit, before Hydratech came into existence, I would prove to skeptical customers that even hitting them with 25" of vacuum wouldn't satisfy what they were looking for. I had a 25 gallon air tank from a failed air compressor that I would suck down to 25" of vacuum using a RobinAir VacuMaster HVAC vacuum / evacuation pump. The tank was just right to put a heavy quilt down in the trunk, load the tank into the trunk, then run a 3/8 vacuum line from there to a vacuum gauge temporarily taped to the windshield, then to the vacuum booster. I would hop in the passenger seat while the owner would drive, watching the vacuum gauge reading while making a short trip down the road and back to evaluate what a large amount of high vacuum applied to the vacuum booster would produce in a real time scenario. Most would always start their conversations by saying their brakes aren't working right due to a lack of vacuum from their lumpy cam. Hooking this oddball arrangement up only took a few minutes to do, and illustrated to the vehicle owner that a mini booster provides exactly that -
mini boost. The other quick way to do it was to simply set the tank next to the vehicle using a 30' length of hose connected to the customers vacuum booster (if they had a big sound system in the trunk or other items where we couldn't fit the tank in the trunk). That way they could pull forward and backwards up to 20' or so to evaluate any difference in braking with high vacuum temporarily feeding their vacuum booster. The high vacuum artificially supplied always helped a little, but nowhere near what was expected. Out in the field across town, we could alternately take the 40' hose and connect it to a dead stock vehicle with strong vacuum, then connect the other end to the muscle car in question, again allowing a few heavy applications of the brakes in the short distance they could back up and pull forward, illustrating that higher levels of vacuum wasn't always what they expected would help their braking woes.
My further testing on many vehicles led me to find that downsizing the customers MC bore size would make some of them happy by increasing the hydraulic leverage / brake line pressures for the same given amount of general brake pedal pressure applied, though at the expense of increased pedal travel. This was the cheap easy fix that most customers liked. Beyond that, it would then lead to installations of semi metallic specialty pads and shoes with much more bite, but then producing the side effect of heavy brake dusting of their custom wheels, along with reduced pad / rotor life with noise and squeals. It wasn't unusual to have a customer go through a set of pads in only 5-6k miles (!). Of course that then led to upping the ante into the installation of larger aftermarket brakes at the wheels, but wheel sizes kept being a limiting factor 30 years ago. Some applications would allow for the installation of calipers and wheel cylinders with slightly larger pistons - that always helped considerably by increasing the caliper clamping force / brake shoe apply actions at the wheels. Again, downsizing the MC bore size, along with installing pads / shoes with more bite usually satisfied most back then. Some customers would have us show them actual line pressure gauge readings - engine off, engine running, then with the 'artificial' high vacuum levels supplied to their mini boosters, so that they could really put their eyes on what and why the causes were for their braking deficiencies. We typically observed about 700-800 PSI with the engine off, no vacuum in the booster, with a healthy brake apply. Firing the engine up and revving it a few times to draw down a decent 15" +/- vacuum in the booster would typically add about 150-200 PSI to the baseline engine off gauge readings. Supplying the mini booster with 25" of vacuum surprisingly would only increase the line pressure readings an average of 75-100 PSI more as compared to a 15" vacuum supply to the booster. Everybody usually expected more with a gain of 10 more inches of available vacuum, but the gauge readings didn't lie, proving that most everybody always wanted to blame their lack of proper braking on the big cams not producing enough vacuum. Getting some customers to understand that it all has to work in harmony in a well balanced system sort of didn't set well with some that were just dead convinced it was all about vacuum or that their mini booster must certainly be defective.
Soooo.... Yes, I'm all about doing various different things to help with braking such as what you are discussing here, only adding my experiences of vacuum versus line pressure testing versus real time road testing. My findings where that if you even only had 12" of vacuum to work with, you could still tune around that with smaller MC bore sizes and increased friction capabilities or increased clamping force (with larger caliper piston sizes). Vacuum reserve tanks always seemed to only help marginally, with some customers complaining that their cash outlay to have us install vacuum tanks and pumps didn't meet their expectations after returning from a road test, as they were just so utterly convinced that it was vacuum vacuum vacuum... (sigh).
Why all of this? Reading your posts just reminded me of many years gone by testing all kinds of different means of accomplishing appropriate braking in all kinds of different builds. I went through everything from soup to nuts always chasing the ever elusive mysterious lumpy cam / lack of vacuum scenarios with countless customers / builds to find that mini boosters provide exactly that - mini boost, no matter how much vacuum you managed to supply to them. Notice I haven't said a single word about hydroboost in this thread, because that isn't my point. Of course hydroboost provides higher line pressures, hence increased clamping force resulting in better braking, but I have tuned in countless vacuum boost equipped scenarios by using the common sense fixes I mentioned above. Yes, definitely sort out your vacuum systems well, just keep in mind it doesn't produce the increases in real time braking that most are looking for, leading to some disappointment, so bear in mind that other mods will have to compliment your vacuum mods to produce a well rounded braking system that meets your expectations. There are just WAY too many people out there that have gone through all kinds of attempts to chase vacuum around - heck, I even ran into a guy that had tandem reserve vacuum tanks in his attempts to chase the ever elusive "lack of vacuum".
Just my .02 cents - I'm sure the majority of you know all of this, but there are just so many people out there spending hundreds of dollars on the latest vacuum pump designs and vacuum tanks, when they really should have put that money into the actual brakes at the wheels, different master cylinders, and adjustable proportioning instead (!). Playing around with the different pad compounds Wilwood has available will work hand in hand with getting your vacuum project sorted out - be prepared to try out a few different pad set swaps to evaluate. A swap out to a 15/16" Wilwood MC may also be a direction to potentially evaluate in your build.
Thanks for letting me ramble on a little!
Brian Hobaugh SCCA National Tour June 2014
First Hemi 'Cuda Convertible Ever Built
Short clips: Goodguys Pleasanton autocross and pit videos
Vacuum brake system design - switch input needed
Aftermarket mini boosters
Curious how it worked out?
Linear Mode
