I'm not even close to an expert on this subject, but I'm working this problem on my own car, so I've been thinking about this recently.
The question of possibly
using a drum brake prop. valve brought back a lesson, hard learned, from back in the day.
Vehicles equipped with front disc, and rear drum brakes often use a metering hold off valve in their braking systems. It's generally co-located in the proportioning valve housing.
It has multiple purposes, but the main one is to allow the front and rear brakes to energise at the same time. Without it, the front disc brakes would apply sooner than the rear drum brakes.
The why of that goes like this...
When you press on the brake pedal, the caliper pistons move 1 for 1 with brake presssure.
Rear drum brakes, however, have strong springs on the brake shoes that return them to the rest position when you release the brakes. These springs will resist the motion of the shoes at light pedal pressures. This will delay the application of the rear brakes. In fact, at light pedal pressures, only the front calipers would be doing any work, because the rears would have to overcome that spring pressure to move enough to contact the drum.
Sooooo, on drum brake equipped cars, there's a dingus in the prop valve that would time the system, and adjust the front/rear work levels at light pedal pressures.
The problem is when you use that style prop/metering hold off valve on a vehicle that has rear discs...it will make the rear brakes uber sensitive at low pedal pressures.
For those of us that do a lot of winter/snow driving, that spells trouble with a capital "OH SHIT" on ice at high speeds.
My own experience has also been that it will hold some ~small~ amount of line pressure on the rear circuit.
It only holds a couple of psi, (certainly less than ten psi), so it's hardly noticeable, however, it's there, 24/7...especially when the system is warm.
Backstory, (for those who care
). I found this out the hard way back in the day playing z-cars. The rear disc brake conversions we were doing would eat rear pads. In addition, the rears would run hotter, and stay warmer longer than one would expect. The front/rear balance was ~adequate~, but we ate rear pads any time we really pounded on the cars. (Keep in mind, these were v8 swapped nissan z cars, we drive hard, and we live in the mountains)
We didn't have the intraweb back in the day, so we had to figure shit like this out ourselves.
I used to call the various suppliers, manufacturers, and anyone who would listen to ask questions, and every now and then we would get lucky. One day, I was abusing a retired GM project engineer about doing a conversion using corvette goodies, and he turned me onto a retired Lock-heed engineer in our area that helped design the systems.
He asked to see the set-up we were running, and put some really cool brake pressure gauges where the bleeders go and we went for a ride. Yep, you guessed it, ~4 psi , (with the pedal released), ALL the time to the rear brakes.
It was an education, listening to that man! (Sadly, he's passed, 8 years ago...
One thing he did teach me is to understand the whole system before I started tampering with it.
I have tried to follow his sage advice.
I'm doing the abs delete on my own car, and I had a bunch 'o questions as well.
(searching the topic just gave me a headache, as the information contridicts itself many times in the threads I found, both here, and elsewhere on the intraweb....
Has anyone here ever actually measured the brake pressure, the knee point, and the actual split from the factory prop valves available?
I see a lot of people describing front/rear balance as a function of lock-up.
There's so many variables in the equation, that I'm not so sure that's a good definition of the actual
hydraulic pressure settings.
The front/rear brake pad materials, tires, suspension settings, battery relocations, road surface etc. will have a profound effect on the effective
brake balance. It's one part brake leverage, one part tire adhesion, one part road surface. (Etc) These all effect weight transfer.
Intuitively we can see with better tires/road surfaces, bigger front brakes etc, we get more weight transfer to the fronts, and the rears will lock more easily/sooner.
Change one variable, and you change that threshold where the rears will lock first.
For example, even the grade of the road effects brake balance. I find myself hazing the rears going downhill when I'm late for work, but going home, uphill, same road, same car, same driving style, I never hear the rears even when I brake up to, and past the apex. (Boost/Brake...oh...yea..
I'd love to have a thread in the how to's that had empirical evidence of what works on these cars!
On a side note, I've been looking at the various master cylinder choices, and, as a point of interest, Mitsubishi has a different set of internals in the non abs version of these master cylinders. Anybody know why?
Oh, yea, the original topic...(
) I'm running a 1g, awd, non abs prop valve in my 92 vr4, and it works ~reasonably well, in most circumstances, and I'm pretty particular about things like that.
Still.... I'd love just a schosche more rear bias...