Brake Help Request

Link to Build Thread

In the interest of getting more eyes on it, I'm starting a separate thread than the build above.

Long story short, I'm converting a 92 Dodge 3/4 ton truck with the Cummins from rear drums to discs using what appears to be a nice kit from Far From Stock.

My plan is to put a proportioning valve in line bypassing the box below with the wires going into it. (the circuit for the front brakes will remain untouched.

Does anyone know if that box has a residual valve in it? What does the sensor operate (cruise?) Is this a dumb plan?

I also have a 2lb residual valve I'd like to put at the back of the truck.

This contraption is some sort of pre-historic ABS. I hope to bypass it, and put the residual valve in its stead. Again, Stupid? Good idea? Let me know what you think.

Why do you think you need a residual valve?

I wouldn't expect the block to have a residual valve, especially for the rear on a rear drum brake system. 

If that sensor is like most vehicles, it will trigger the brake warning light on the dash. This will happen when the pressure differential between circuits is enough to cause the internal valve in the block to move, like if a line fails when applying the brakes. 

2nd what No time said and I don't think discs need a residual pressure like drums do.

Older disk/drum setups had a pressure hold valve that would keep the front brakes from getting pressure until 50psi or so, to allow the rear wheel cylinders to overcome the return springs so all four brakes applied at the same time.  I do not see one of those in that block.

If you eliminate that block you will lose the prop valve and brake warning light functionality.

The "contraption" is rear wheel ABS. The sensor is on the pumpkin. I think its OK to bypass it. Or maybe not if it still works.

Comments appreciated, everyone. I felt I needed a residual valve because an engineer buddy who has been down this road said I needed one. 

I remember hearing they were only needed for hot rod type situations where the MC was at or below the level of the calipers. I'd be totally cool with leaving it out if it's not necessary.

I don't know if the ABS thing works or not. I'm thinking of simplifying, as it does need to be bled. 

All opinions welcome.

RWAL is best thrown in the nearest dumpster.

I have never found a need for residual pressure valves unless it is an underfloor mounted master (think 30s street rod) or the car has a hydraulic handbrake (pushes fluid backwards until the compensation port closes)

Residual valves are more commonly used in applications where the master cylinder is below the wheels, like street rods that mount the master on the frame.  In drum brakes, the weight of the fluid can cause the pistons in the wheel cylinders to pull back further than just resting causing unpredictable differences in brake application.  Since you're not putting the master below the wheels, I wouldn't use one.  Having the master cylinder above the level of the wheels is like it's own residual pressure valve.  The fluid will want to flow downhill.

That prop valve you have might be the Delphi.  Sure looks like one.  If you can find us a part number we might be able to help.  Impala SS guys found a hack.  When the B-body went to 4 discs, GM never changed the master or prop valve, so the rear discs got woefully low pressures.  The (mathematically calculated by a brake engineer, road tested, proven safe and correct) solution for us was to gut the prop valve.  I can't suggest this for you since it would require a good bit of research, but it's a super simple mod if you have the right prop valve.  There is a big cap bolt on the end closest to the firewall.  If you pull that off you'll see a white plastic piece.  You can grab it and pull it out with a little effort and you have effectively removed the proportioning part of the valve.  In the case of our brakes, it was just right and required no plumbing.  It also kept the factory brake warning light functional.  The whole mod took about 2 minutes.  Not sure about yours.  I can't recommend that for you without doing the maths first.  Just floating it as a potential option.

That box does not have a residual valve in it.  The wiring is for the brake warning light.  If one half of the master fails you get pressure on one side of that box and not the other.  The pressure forces a little piston to the side and makes contact on that switch which turns on the dash warning light.  It is a brake malfunction indicator switch.

The cruise cancel just operates off the brake light switch on the pedal.

Hey, Thanks, everyone, I was just preparing to bump this as a hail Mary on blocking my stupidity. 

In particular, Curtis, Thank You. Most helpful.

I'm leaning hard toward deleting the ABS, gutting the prop valve as suggested above, running the adjustable Wilwood Prop I have, and having at it. Cool