rhd racer wrote:The reason people usually fit Wilwood pedal boxes (and it will have a value when removed) is to allow you to change front to rear bias by adjusting a pushrod
This is for adjusting brake balance not bias - correct brake balance can be achieved through correct sizing of master cylinder(s) and is important to get right
Brake balance affects the front vs rear brake line pressure for any given pedal travel and is usually calculated using a max braking potential for the car. Its worked back from the max brake effect at the front vs rear tyre contact patches, to give you the correct pressure ratio front vs rear at the master cylinder(s)
Think of it as a graph with two straight lines at different angles - pedal travel on the X Axis, brake pressure on the Y Axis. The balance is the difference in gradient of the lines
if the rear brake graph is steeper than the front = more power to the rear brakes than the front
single line = same power front and rear
if the front brake graph is steeper than the rear = more power to the front brakes than the rear
Almost always you have more braking force on the front axle, which (assuming brake discs and pads are similar F-R) requires a higher line pressure
Whilst doing the mods to the lines, you could fit a brake bias (sometimes called a compensator) to allow you to fine tune
This is for adjusting brake bias
Consider the above graph again, this time with a dog-leg in the rear line, so initially a reasonably steep gradient, then at a certain point it changes to a flatter one
The intention of a brake bias valve is to prevent the rear from locking up as weight shifts forwards under heavy breaking. So up to a certain braking force or brake pressure the front and rear brake pressures increase in a straight line, above a certain pressure the front pressure will continue to increase in a straight line, the rear will increase less quickly
Brake bias valves come in two common flavours
1) adjust bias % with a fixed point - this type of valve adjusts the angle change between the two areas of gradient
2) adjust both - you can adjust the amount of bias and the position where it occurs
the third is less common
3) adjust bias point with a fixed % - this type of valve adjusts where the dogleg occurs along the line but the angle is fixed
Here's a graph from pelican forum which does a reasonable job of explaining it
They refer to it as a proportioning valve, and use the term "change over pressure" for the point where the bias takes place
They have mislabled the axis - the X should be pedal travel and the Y should be brake line pressure - the axis as they have it would be for a single line (the red one) and not two lines (front and rear) like they've shown
You can see here the red line is the rear brake line pressure, the blue is the front - there is a difference in angle initially which is due to the difference in sizes between the front and rear master cylinder. At the "change over pressure" point, the bias valve starts to reduce the pressure gradient on the rear line, preventing the rear wheels from locking up as the weight on them reduces under heavy braking
if you're not happy with the brake pedal box you have then remove and sell it on ebay, they're £600 ish new, master cylinders are £75 each, they hold their value pretty well. Just be aware if you're changing for cosmetic reasons i doubt you'll get anything better performing for under £2k, the 600 series are a really nicely made piece of kit, although it sounds like you maybe have some headroom issues and that's the reason for change?
I have the same pedal box in one of my cars and its served me really well
There are plenty of online calculators for brake MC sizing - you want one that takes into account axle weight, centre of gravity, caliper piston size, brake disc effective diameters, pad height, etc. The more of a pain in the arse it looks the better it probably is. All of the info is googleable or measurable so its not a big deal to do. Get your MC size right, and then starting with a bias valve set to "off" (or equivalent) you can experiment somewhere safe to bring the rear line pressures down a bit
Re: lines - I'd say its not worth the potential headache with modifying the rear lines, if they've been flexing they could be damaged or work hardened up and down their length, so better to just replace them. Then you know they're from kunifer not copper, clipped properly, etc., etc.. the materials are cheap, if you get someone else to do it for you and they have the right tools its not going to take them that much longer than modifying whats already there. Depending on how accessible it all is, probably worth doing the whole lot so they fit the new pedals correctly too. Might also be worth doing the clutch line as well (assuming its hydraulic?) and bleed the whole lot with some fresh fluids so its as good as it can be
HTH