I have a 73 jimmy that i'm converting to hydroboost currently, i'm also looking at adding an adjustable proportioning valve for the rear brake system. My question is, can I install an inline adjustable valve after the factory valve? I was going to buy a jegs -
http://www.jegs.com/i/JEGS-Performance-Products/555/63022/10002/-1 valve and just add it after the factory valve.
I don't mean to sound preachy, but I would not recommend the installation of two proportioning valves in series. The back pressure produced by the downstream valve will - at some point - almost surely affect the balance of the springs and spool operation in the OEM one. And that in turn will make the downstream unit behave erratically. IOW's they'd be stepping all over each other.
The primary functions of a BPC valve are metering (i.e. by initially limiting the flow of hydraulic fluid to the front circuit until the rear brakes can make frictional contact with the rotating assemblies and rear circuit hydraulic pressure begins to rise).
Once that design pressure in the rear circuit is met, the spool in the BPC valve overcomes spring pressure and shifts -allowing flow to the front brakes. Also at that point, the BPC valve becomes a proportioning valve and it's primary function is to limit flow pressure to the rear circuit. During periods of heavy deceleration, a certain percentage of the vehicle's weight that is normally carried by the rear wheels shifts to the front wheels. The resultant loss of downward force at the rear tires makes them more apt to lose traction with the road surface and skid/fishtail. But, because the proportioning valve is limiting the degree of hydraulic pressure in the rear circuit, the tires can continue to roll on the road.
Proportioning Modifications
We could start this section by clearly stating that you should not modify your proportioning valve. But, what fun would that be? In all seriousness, making changes to the proportioning valve to effect brake bias should be left to those with the proper tools and measurement devices, but if you have tweaked your vehicle beyond recognition, this may be your only solution to restore a sense of proper bias to your braking system.
We’ll start here with three of the most basic rules regarding proportioning valve installation and selection.
1. If you have the deeply-rooted need to install your own adjustable proportioning valve, be advised that they should NEVER be installed if the factory unit is still in place. Proportioning valves in series with one another can do nasty, unpredictable things!
2. If you have the deeply-rooted need to install your own adjustable proportioning valve, be advised that they should NEVER be installed in-line to the front brakes. The effect would be to make your vehicle rear-biased before you could say “terminal oversteer.” Front brake line pressure should always be left alone – only the rear pressures should be considered for proportioning.
3. In all cases, the basic brake system balance needs to be close to optimized to start with. This is the only way that a proportioning valve can be effectively utilized. You should never assume that simply adding a proportioning valve will address all rear-bias conditions, as even the best proportioning valves must be well-matched to the target vehicle.
