Bogus brake description

On this week’s show I heard the brother’s explain to a caller how modern cars have a dual brake system. So far so good, but then they went one step too far and suggested that the callers front brakes might be the failing half. I’m pretty sure that all the modern dual braking systems split the braking into front-left/right-rear and front-right/left-rear pairs. They guy may only be getting half his brakes, but it should always include at least one front and one rear if, in fact, it’s a failure of one brake circuit.

I know hondas for sure have the setup you describe with diagonals sharing a circuit. But the majority of cars, particularly earlier ones, were split front and back.

Many still are. (F/R split)

i dont think so.

i had an old F 250 and the brake line rusted out, so i got a cheapo pair of vice grips and crimped the line to get home with.

that line only went to the rear brakes.
so mine were split front and rear.

I’ve been curious about how that arrangement stacks up and the reasoning behind it. Normally, the bias favors the front brakes. What happens when you get diagonal control only? Doesn’t the front brake being biased more cause a pull? How is diagonal failsafe any better than the axle based design?

Well, honestly, you learn something new every day. I remember the diagonal split on cars back in the 1970s, and never had reason to look too closely after that. I am amazed to learn that some cars don’t do it that way. According to one web based source, front wheel drive cars use the diagonal split and rear wheel drive don’t, but that can’t be completely correct because I know I saw it on either Chrysler or Ford cars in the late 60s and early 70s. Those were all RWD.

Anyway, interesting to learn something new.

It may have something to do with the evolution of brake systems, from all drum to disc/drum to 4-wheel disc brakes. There is a proportioning valve to control brake fluid pressure to each half of the brake system.

Thus it is possible that either a diagonal system or a front/rear system, combined with a suitable proportioning valve, would provide the safest emergency stopping. It would depend on the car and its brake properties. Just my own theory.

Except that the diagonal system would need two proportioning valves and more line since both halves must service a rear brake.

Well, if you lost both front brakes, you’d have very little braking left from the rear axle. Losing the rear brakes might not even be noticed. I suppose that losing one front and the opposite rear might give some pull to one side, but that’s probably the least of multiple evils.

Every car I have has the diagonal system, including an 86 Toyota. It does have two proportioning valves and it does not pull when you loose on circuit. The fact that it doesn’t pull is one of its main advantages. There are a few cars that use the Brembro or Volvo system where there are two pistons on each front caliper. Each cylinder on the MC feeds one piston on the front calipers and one rear wheel, three wheels always have brakes.