Andy, I have the fitting to go in the line to remove the adjustable proportioning valve and will most likely get it changed today. I'll go over the lines to see if there are any kinks, it has 3AN flex lines. When I get some help I'll check the pressure again.

CMcAllister, the fronts are Strange race brakes I bought from Hemi Fred years ago, the only other option I see for calipers on these brakes are 1.75" pistons. I can go to a larger piston on the rear Wilwood's if needed but their tech didn't think I needed to. Once I get the pressure figured out I'll see how it stops and make any needed changes.

Thanks guys, this brake stuff is greek to me. lol

Even if a line is kinked but not totally closed
off that pressure will finally equal out on that
line..in a period of time.. same thing as a prop
valve... this just hyd 101

Consider 500psi of fluid pressure at one 4-piston caliper.

With 1.38 pistons, the force exerted on the pads is 2960lb. total.
With 1.75 pistons, the force exerted on the pads is 4800lb. total.

It would seem that way but it isn't; when computing the clamping force of multi-piston rigid calipers you only use half the caliper pistons. IOW, only one side applies force, the other side acts as a backup. I know this is counterintuitive but that's the way it is.

That's where I took mine.

I don't think I would want more stopping on the back, street or strip. Once it starts sliding the car tends to swap ends. Did you bleed that MC good? I would start the guage at the MC and work down.

Without knowing weight bias, you can’t really say it should be either way. I can tell you that my own car, that had countless hours of experimenting on the brake system, was as well balanced (brake wise) as any car I had ever driven. It was a mish mash of odd parts that worked in orchestra on a heavy car that had no business going as fast, or stopping as quick as it did. Each front wheel (26” tall Eagle front runner) had 1100 pounds static weight, while the rear (31” 10.5W MT) were at 750 pounds static weight.
The car had 12”dia vented front rotors with wilwood large piston calipers. The rear of the car had 10” drum brakes from a cuda or some such car. The master cylinder was what you would find in a late 80s caravan. Those master cylinders have no built in bias because they are designed for a cross braking application. My adjustable prop valve was in the rear brake circuit. I adjusted out some rear brake until i could skid the car evenly at speed.

I was under the impression that the aluminum master cylinders that are popular on our cars don't have any bias built into them, but they do have two different volumes for each side of the reservoir which is why the rear port is supposed to go to the larger volume system on the car.

Understood, the proportioning works an adjustable orifice restricting flow, thus initially pressure too. In a closed system like brakes the pressure will eventually equalize on both sides of the orifice.

Right now I just want to find why my pressure isn't where it needs to be on the initial pedal pump.

The proportioning valve is out and the union is in. Have to work tonight so hopefully tomorrow I'll re bleed the fronts and see what I have.

Thanks, Justin.

When you say a master cylinder is biased, are you saying you get different pressure out of one line VS another? Is that possible?

I don't think it is.

When a car is designed, the size of pistons front or back gives the manufacturer the calculated brake capabilities.

And that factory valve that most call a proportioning valve is not that at all. It is a shuttle valve that gives you the dual/failure mode braking if front or back leak/fail. This is not done in the master cylinder.

I've been wrong before, so I am correctable, eh?

Yes you can get different pressures out of the master cylinder. I understand your query. Since the bore is the same, it would seem that output would be the same. The way you can get a pressure bias is through piston surface area. The rear most piston has a centered semi floating rod that deducts from the overall surface area of that piston. The second piston has no rod, or less of a rod. More surface area= less pressure.