Brake Booster Delete - Dual Master InstallI've hated my brake booster since I installed my bigger brakes and race engine. The cams create vacuum pulses as I blip the throttle down the gears so my brake boost assist fluctuates. This means I can feel the pedal gain and lose assists whilst hard under brakes. I also find upon initial application I get a large boost assist as the booster is in vacuum, only then to loose the assist halfway through the brakes. The booster had to go, plus it is heavy and ugly.
My plan initially was to reuse the stock tandem master and just remove the booster. Halfway through this I figured I'd go all out and put in a proper dual master with an adjustable balance bar. Here is my initial design:
First some background:
I don't have stock brakes, so if you are copying this you'll need to factor in the piston sizes as well as rotor sizes. I run Wilwood Dynapro 4 pot calipers (same area as stock) on 11.75" front rotors, and NA6 stock calipers on offset brackets on 11.44" rear rotors.
This effectively gives me slightly more rearwards bias over a stock setup, but also gives me a decent leverage ratio as the calipers are acting on the rotor from further out.
I used to run with a Wilwood proportioning valve. Fully open I'd get my rear squirming under brakes. I found the balance point approx 15% closed.
The stock tandem master is a 7/8" piston. The front and rear are separated via a floating piston, but for pressure calcs it ends up being equal to a single 7/8" piston. When you replace this single master with dual, you have to size the masters at 75% of the diameter. Area (pressure) = pi * r^2. This sizes the replacement masters at 0.656" but since I need some less rear bias, I chose a 5/8" Master for the front, 0.7" master for the rear.
Pedal Box ModificationsOnce the brake booster is removed, there are 4 mounting points that the masters and the pedal box can mount through. My plan was to use the existing holes, which are the exact same centers as the master cylinder mounting points and just so happens to also line up nicely with a standard 2.5" - 3" balance bar. This means the only modifcations to the firewall and pedal box are to dill some new holes to clear the new locations of the masters.
These same holes also need to be drilled into the firewall. I think the diameter is 28mm but this will depend on the master cylinder used. The holes in the pedal box unfortunately weaken the structure a lot so it will need to be braced to get back some strength. Without a booster I'll be putting more pressure on the pedal so everything needs to be strong.
The balance bar itself gets welded into the pedal at the same pivot point as the stock push rod fixing. Once again you need to drill a hole of suitable size to snugly fit the outer sleeve and then weld it in.
If later I find I need to increase the pedal ratio from 4:1 (stock) to 6:1 then I'll need to move the top pivot point down around 20mm and shorten the pedal the same amount. This would give me a 150% increase in pedal pressure with the downside of an addition mount of pedal movement. For now I'll keep everything at the stock ratios.
I added some bracings to the sides and down the center out of some 2mm steel plate. I didn't take any photos of these but will try and take some in the car so you can see what I did.
NA8: N/A 200whp | Haltech | Skunk2 Intake | S90 TB | RCP | 5 speed c/r dogbox | 4.78 diff | AST Shocks
Barbs L: 64.12 | S: 58.62 | Collie: 49.72
