Brakes Part 2
Rather than design a brake system from scratch, Rodders normally chose to try to get the units that came with their axles to work together. Not a problem providing you have a basic understanding of the principles involved. Under hard braking the weight transfers to the front of the car, normally 90% to front. The braking system is balanced to provide heavier braking effort to the front and this is accomplished by having larger bore cylinders, be it drum or disc, to the front. You should never have larger cylinders or more braking effort to the rear, as this will cause the back end to lock and skid, becoming uncontrollable under heavy braking conditions. The discs are far more effective than drums as they provide not only a larger braking area but also cool a lot more effectively preventing brake fade.
The amount of force applied to front and rear brakes is controlled by the bore of the master cylinder, but also relevant to the mechanical leverage provided by the brake pedal and its effective ratio. For any set ratio the larger the bore the less pressure output, but also the less the pedal will travel. The idea is to try to get a balance that gives a good pedal feel but with reserve travel. Preferably the brakes should be full on by ¼ to 1/3 of pedal travel.
On all disc system it requires a minimum 1000 psi to operate effectively. Looking at a production Jag, with a 5.5:1 pedal ratio, 15/16th m/c bore and a pedal load of 100lbs, you will only produce about 575 psi. So where’s the rest of the pressure to come from? Normally a servo is used to multiply the inline pressure to what’s required hence why it’s hard to stop a car when the engine has stopped running. Unfortunately Rodders, being Rodders, can’t leave their engines alone and fit lumpier cams which reduces the vacuum required to operate the servo. The brakes are normally fine at mid to full throttle situations, but town braking, after only one or two applications, can become difficult. This is usually far more apparent on an automatic gear box, as on a manual you tend to blip the throttle, or build the revs up sharply under acceleration, which overcomes this problem. There are two options. If you are still in the building stage then fitting twin master cylinders split front to rear, will provide all the pressure you will ever need. A wilwood pedal assembly with a 6.25:1 pedal and ¾ cylinders will give 1349 line pressure without the need for a servo. If you are, however, trying to solve a slow speed braking problem on an existing build, and you have installed a performance cam, then you can supplement the low speed vacuum with an electric pump. These are available new from MP Brakes or similar companies. For those of us on a budget suitable units can be sourced from Volvo 760T, V70 and S40. Due to the turbo on the first unit and strange cam set ups on the other two they require vacuum supplementation. It’s wired so that it automatically switches off when a set 18-21 vacuum is reached. To allow it to do this you’ll need to tee into your engine servo line with one side to the pump, one to the servo and the line to the engine needs to have a Lockheed one way valve LV15069 installed. Without the valve the pump will run continuously as it’ll try to create a vacuum in the engine as well as the servo.
So in another scenario, you’ve transferred all the braking system from one car and yet the brakes still don’t work as they should. Well have you installed it EXACTLY as per the manufacturers spec. For instance altering the angle that the master cylinder sits relevant to the ground may mean it’s now impossible to bleed it correctly. Have you changed the pedal ratio by altering the original or making your own? Just ¼ inch can affect the entire system, especially if taken from near the master cylinder end. Most OEM systems use between 5 to 6:1 ratios, Rods may require as much as 7:1, depending on master cylinder bore. The effective ratio is worked out by measuring from the centre of the pivot hole to the point where the master cylinder attaches. Then measure from the pedal pad, in a straight line, to the same pivot point. Divide the first into the second and you’ll have your ratio. Remember it’s got to be in a STRAIGHT line, ignore the shape of the pedal, particularly an under floor one as all forces act perpendicularly. This is also important at the master cylinder end, the rod must move away in a straight line, any deviation will lose braking pressure.
The higher the mechanical advantage of the pedal, the greater the braking force generated but at the cost of an additional pedal travel, it’s a delicate balancing act. On British and European systems, the master cylinder is usually sized at 5/8 for drums, ¾ for disc/drum and 7/8 for all disc although, dependant on pedal rations and slave cylinder sizes these can increase by up to 3/16 inch. American systems are normally much larger.
Back to those dodgy brakes that SHOULD be working per manufacturers specifications. Have you moved the unit to under the floor, because this brings further complications. If the calipers/slave cylinders are level with, or above, the master cylinder reservoir, you will have to fit residual valves, a 2lb inline for discs and a 10lb for drums. Don’t get them the wrong way round as a 10lb used on discs will lock them on. These are needed to prevent fluid draining back, under the force of gravity, causing air pockets in your brake lines. If you can bleed your under floor brakes and they are firm until 24 hours later, it’s highly likely you’ve got bleed back. Stick in a Jag rear and hot rod rake and I’ll guarantee you have a problem. It may even be so bad that the residuals alone won’t solve it. In this case fitting a remote reservoir filler well above the height of the rear calipers will solve the problem. You have to, if using the standard master cylinder reservoir as well, ensure that it is completely full of fluid as any air space will still allow bleed back. If the master cylinder you have chosen was normally used as an under floor it will have a residual valve built in. You cannot use further inline residuals without removing these first.
This brings me to the final point to watch (PHEW!!). If you want a front/rear split circuit use a cylinder designed for that purpose. Likewise if you want to use a diagonal split circuit use a cylinder designed for the job. On front/rear split systems the larger port will go to the front.
I once measured up a front/rear split master cylinder, both bores were the same with the same swept volumes, so I chose to set it up as a diagonal split. When MOT’d it passed but was giving a lower reading on one split to the other. After trying Jaguar specialists, Jaguar Direct and Lockheed tech with no solution, I spoke to Girling technical. They confirmed that, according to the engineering drawings, the cylinder gave a 58/42 front/rear braking bias although they had no idea how it was achieved! Conducting a specialist test showed that one rubber was designed to allow pressure to bleed back into the reservoir. Lesson learnt!!
So to summarise, getting good brakes is all about balancing good line pressures against leverage vs pedal travel. You can get to the bottom of any brake problems – it requires checking each section is operating at optimum and a determination that you won’t let an inanimate object beat you. Good brakes, and steering are the cornerstones of a fun, usable and safe car.
Kev Rooney.