Soft Pedal when Running

[slideways]

Im redoing an 83 K10 that i assume has JB5 brakes. This is my first Chevy. The only sticker in the glovebox talks about camper insert ratings. Nothing else anywhere. I have all new rear brakes/drums, new calipers, new MC bench bled. I am using a Wilwood adjustable prop valve with 2 inlets 3 outlets (one dedicated to rear and split front). I have run unbroken lines to each caliper and the rear Tee with SS soft lines. Everything bled out ok. The rear took a long time. Front calipers are C clamped closed for now. With the truck off i get a nice hard pedal. Zero leaks or seeping. With the engine running the pedal feels normal if not a little soft but goes down and has a hard stop to it. Not just against the hydraulic pressure. The check valve in the booster is working correctly and the line doesnt leak air. When you shut the engine off the booster gives about 1 1/2 pumps of vacuum Whooosh and gets hard which is what i've seen in other cars mostly.

This prop valve has a bracket that goes behind the MC which makes it stand off about 3/16" from the booster. I played with the brake rod at the pedal lengthening thinking maybe it wasnt getting full travel into the MC but similar results.

[bd]

Are you chasing a soft brake pedal complaint only with the engine running?  Power brakes generally present a "softer" pedal feel than do manual brakes.  How well does the truck stop?  Any pulling to one side or the other with brake application?  What do you mean by, "Front calipers are C clamped closed for now?"  Did you gravity bleed the brakes using DOT 3 fluid?  What method did you use to adjust the rear brakes?  How much free travel remains in the brake pedal pushrod before the rod engages the M/C piston?

To help determine the correct brake RPO code, answer the following questions.  How thick do the rotors measure?  What are the measured width of the shoes and the diameter of the drums?  Is the power booster single or dual diaphragm? 

[slideways]

Only when running yes. I understand that the brakes will be easier than with the engine off. They pump up nice and hard with the engine off. My issue is that when running they go down pretty soft and then stop about halfway. Not like you’re pressing against hydraulic pressure like they’re locked. Like there is a mechanical stop.
The truck is immobile on jack stands. I will be pulling the front axle to rebuild it shortly. I just wanted to replace the brakes and verify function at this stage. I have a c clamp holding the pistons completely closed on each caliper. I did the traditional RR LR RF LF bleeder crack/pump method for bleeding. I don’t know how I can measure free travel with the MC bolted up. With the pedal retracted the rod will wiggle free in the cab indicating to me they are not preloaded on. Dual diaphragm. Rotors I believe are 1.25”thick and rear shoes are 2 3/4” 11” drum

[bd]

I agree, your truck has the JB5 brake option.  It sounds like you have air in the hydraulic system as well as may need to readjust the rear brakes.  Perform a gravity bleed while taking precautions that the master cylinder doesn't run empty using DOT 3 fluid from a freshly opened original container.  Back off the parking brake adjustment completely and readjust the shoes.  Always adjust the parking brake after the shoes are properly adjusted.

[slideways]

I’ll attempt a gravity bleed. Today I measured that my MC pushrod has 1.5” of travel at full stroke at the booste. I also verified that there is about 1/16-1/8” of free travel before the rod engages. I plugged the MC ports and with the truck running the pedal was solid which I guess means at least I have the MC bled out right. My rear brakes I adjusted when I assembled by just getting a bit of drag. No parking brakes connected yet

[bd]

With the drums squarely seated on the axle flanges using three reversed lug nuts in a triangular pattern (adjusting windows in the drums) or the wheels (adjusting windows in the backing plates), adjust the shoes until you can't rotate the drums/wheels by hand.  Then back off the adjusters 12-15 clicks of the star wheels.  Assuming the drums are round and concentric with the center pilot hole, they should rotate with a light drag at which point the parking brake can be connected and adjusted.