Braking bias determines how much braking effort is carried out by the front wheels, versus the back wheels. Since weight transfers forwards when decelerating, it's normal to have the front brakes doing more of the work - that's why front brakes are generally bigger than the rear brakes. But if you make brake system modifications, it's quite possible to upset this front/rear bias - in fact, to end up with a completely incorrect brake bias.
And I've experienced the latter! I'd just finished building my Cobra-replica kit car, with all the work done precisely according to the directions given by an automotive engineer. Then one day I drove out of the garage and nearly hit a fence post. This happened when the master cylinder front brake seal scored and wouldn't operate the front brakes. No problem you say - there are always the back brakes. But I had a sore leg from applying so much pressure to the brake pedal to get the car to stop with just the back brakes!
And even if you're not building a car from scratch, a similar problem is possible. For example, what if you've changed the brakes and calipers to larger units, or you've thrown away that spindly little drum-braked rear axle assembly and fitted a disc-braked Ford 9-inch rear end? In these situations it's easy to bleed the brakes and then put pedal to the metal to check them out... to find that suddenly the rear of the car wants to overtake the front.
So bias can be either too much to the front (the rear brakes doing nothing) or too much to the back (the rear brakes locking well before the fronts).
Put simply, what you need is for front brakes to lock the front wheels slightly before the rear wheels lock. (No, we're not talking cars with ABS here!) This means that the car remains in a straight line, steered and stabilised by the rear wheels. With the rears locked up, the back wheels steer the car and the steering wheel is virtually useless.
There are several ways of testing brake bias. You can find a brake dyno, or alternatively grab a friend and some chalk or duct tape and then find a vacant road to do some testing. The marking materials are used on the tyres so that your friend can more readily tell which wheels - the front or the back - stop turning first.
If there is a bias problem (and of course the brakes are in good condition, with no seized pistons or anything like that - see the breakout box at the end of this story) it is then up to you to sort it out or take it to an expert. However be prepared for no help from most brake centres, as changing values in a brake proportioning valve is not something that they normally do - or, in my experience, even want to do!
The Cobra kit car that I built used Jaguar XJ6 three-piston front calipers and XJ6 two-piston rear calipers, with the appropriate XJ6 discs fitted all round. When building the car, a major supplier of kit cars informed me that the correct type of master cylinder to use with the Jaguar brakes was a Mitsubishi Sigma system - booster and master cylinder. Customers of that supplier used an automotive engineer whom I'll call Fred. Fred was responsible for decisions regarding any engineering standards of the kit, including brakes.
So off I went and purchased a Sigma booster and master cylinder, manufactured here in Australia by PBR. Knowing what I do now, the system was probably off a front disc-braked, rear drum-braked Sigma sedan. One thing that I did realise is that I had to remove the residual line pressure valve - it's used in drum brake systems but not in disc brake systems.
After the fright of driving out of the shed with almost no brakes, I thought it was about time to look deeper into what was happening. Especially since I remembered that I had driven the vehicle flat out around Mallala Raceway doing Super Sprints...
I repaired the master cylinder and then put the rear wheels up on jackstands. The Turbo 700 auto transmission was put into Drive, with the engine idling at 850 rpm. It soon became obvious that to get the rear wheels to stop, I needed to use an extremely heavy foot pressure - and all of this while just idling in "D"! No information on the brake bias was available, except a page in a PBR book showed what appeared to be the same master cylinder having different 'crack points', depending on the model of car to which it was fitted.
So what is a crack point?
The 'crack point' is the point at which the front and rear brake system pressures change from being the same front and rear, to where (normally) the rear brake pressure increases in a smaller proportion to the front. That is, as you brake, the front and rear brake lines have the same pressure until you reach the 'crack point', at which point the rear pressure increases in proportion with the front pressure according to the bias ratio of the valve you have. The crack point (such as 'C150') is normally stamped onto the proportioning or bias valve.
Most vehicles have a bias valve somewhere in the system. Some are in the master cylinder, some are in a separate valve body and others use a bias bar or mechanical arrangement. If it is in the master cylinder, then your cylinder looks like two tubes welded together. If the system has a separate bias valve, then the master cylinder has only one tube that looks almost the same diameter along its length, with a separate valve body somewhere else between the front and back of the vehicle. If you have a mechanical set-up, then you have two master cylinders with a balance bar between them, which can be adjusted to put more of your foot pressure onto one master cylinder than the other.
There are several after-market adjustable bias valves such as ones from Tilton, AP Racing, Wilwood and PBR. In these designs, the bias can be varied by turning a knob or moving a lever, which make life easier, however they all appear to have a zero 'crack point' - so be careful. Registration authorities take a dim view if such a valve is within an arm's length when in a driving position, while sometimes it is not allowed inside the cockpit at all.
To vary the bias 'crack point' in a hydraulically actuated bias valve, the internals need to be changed. This generally involves changing the internal spring. While there are often several springs within the cylinder, the largest/heaviest spring is the one that affects the crack point.
If you need to have less rear braking pressure, then the spring pressure in the bias valve needs to be reduced. Conversely, if you need more rear braking, then a stronger spring is required. The latter can be a common occurrence in kit cars or drum brake to disc brake conversions, where all sorts of mismatching of components occurs, even though the registration authorities may only be concerned that the front brakes lock before the rears.
Different bias springs are quite often available from the same manufacturer - for example, in the same model of car but with a different body style. The Mitsubishi Sigma GJ model uses a PBR master cylinder, however the one master cylinder can have three different springs with three different rear brake crack pressures.
The Cobra Again!
With the Cobra I should have started with a Sigma 'wagon with rear disc brakes' master cylinder having a crack point of 550 psi, not the 320 psi version that I was actually using. In the Sigma, the crack points varied between the sedan's disc/drum system, station wagon's disc/drum system, as well as station wagon's disc front/disc rear.
A similar variation can be found with the PBR units in 1977 to 1979 Holdens with disc/drum and disc/disc. PBR also made a P6307 proportioning valve which was an in-line type. It could be found on HQ Holdens, KE20 Toyota Corollas, VH Chrysler Valiants and MS65 Toyota Crowns. Specifications on these can be found in the PBR literature available at brake service outlets.
Other places to obtain different springs to change the 'crack pressure' are the miscellaneous bin at brake repairers (which often contain lots of proportioning valve springs), make use of used motorcycle clutch springs, or you can get a spring manufacturer to make new springs. Then it's a case of suck it and see!
And what did I do?
I worked out that a stronger spring should increase my rear brake pressures. Out with the vernier caliper, bathroom scales and the workshop vice to calculate the preload pressure on the spring as well as the spring rate (ie how many pounds per inch it took to compress it).
Not being able to find any heavier car bias springs around the workshop, I instead found some heavier motorcycle clutch springs to try. In went the heaviest spring I could find, giving me a spring crack point force of 60lb rather than 22lb, although I still didn't know exactly how much rear brake pressure was being developed before or after the spring change.
Back to the jack stands and sure enough, the rear wheels stopped reasonably well at idle. I was feeling a whole lot more confident about being able to stop if the front brakes failed. Out onto the road and my front wheels still locked before the rears, although how much before I don't know as the system still doesn't have enough rear brake pressure to lock the back wheels at the same time as the fronts.
After clamping off the two front brake lines (and finding a safe testing location!) the rear brakes pulled the car up quite well, so I knew I was somewhere in the ball park. Driving around the streets it was a whole lot more confidence inspiring although it would be nice to have done a 'before' and 'after' brake test to prove the point.
Is the car set up to its potential best braking? I don't know the answer to that yet, as it will require more bias spring changes and more testing.
I found no useful advice forthcoming from any brake centre except for one person who referred me to 'Fred', the previously mentioned automotive engineer. I didn't have the heart to tell my informant that the engineer he had referred me to was the source of the misinformation in the first place....