Brake problem diagnosis, hydroboost

[Usanumber1]

Ok, brake experts. I moved out of town, and left the truck at a shop to give it a general mechanical inspection before I drove it to my new home out of state.

general vehicle specs:
6bt
Nv4500
D60 SAS

brake system specs:
GM Hydroboost booster (replaced with reman)
GM PS lines
GM Hydroboost pump
Wilwood MC
Wilwood prop valve
All new brake hardlines
Basically new soft lines at knuckles
All new Ford D60 brake assembly parts


They’ve been addressing the brakes, and they’re locking up, but only the fronts. The prop valve is set at approx 60F/40R. The tech was saying that he doesn’t think it’s the booster because if it was the booster, all of the brakes would eventually lock up, I agree with that assessment. Ideas?

 

[Schurkey]

Why is this posted in "Engine Performance"?

general vehicle specs:
6bt
Nv4500
D60 SAS

What is 6BT and D60 SAS?

What vehicle, what model year?

brake system specs:
GM Hydroboost booster (replaced with reman)
GM PS lines
GM Hydroboost pump
Wilwood MC
Wilwood prop valve
All new brake hardlines
Basically new soft lines at knuckles
All new Ford D60 brake assembly parts

What was the OEM brake system? JB5? Or was this an original hydroboost truck?

Why the Ford and Wilwood parts?

What diameter is the master cylinder bore? How does that compare to the caliper piston bore?

What happened to the OEM holdoff/metering valve--proportioning valve--safety switch (Combination valve)?

They’ve been addressing the brakes, and they’re locking up, but only the fronts. The prop valve is set at approx 60F/40R. The tech was saying that he doesn’t think it’s the booster because if it was the booster, all of the brakes would eventually lock up, I agree with that assessment. Ideas?

Front brakes lock, and you don't think it's the proportioning valve. I'd be looking at:

• The metering valve
• Master cylinder bore diameter
• Front calipers including mounting hardware and internal piston seals and piston condition
  
• Front rotor condition/pad condition
• Rear brakes out-of-adjustment
• Rear shoes/drums condition
• Park brake adjustment
• Rear wheel cylinder bore diameter
• Does this truck have the crappy "10 inch" leading-trailing shoe brakes?

 

[Erik the Awful]

What is 6BT and D60 SAS?

Cummins 6BT diesel engine and a Dana 60 solid front axle.

 

[LC2NLS6]

Is there a little slider contraption in the valve that might be getting stuck - the safety device that if the lines break you still have either front or rear brakes? Replace that thing, or bypass and test 50/50 braking to see if rear work. Pinched rear brake line somewhere during the engine install that isn't leaking or open to enough to work?

 

[Schurkey]

The "slider contraption" is the Safety Switch built into the Combination valve.

It doesn't block anything. It just moves over and turns on a dash light.

The front and rear brakes operate semi-independently because there's two high-pressure pistons (primary and secondary) in the master cylinder, not because there's a safety switch in the plumbing.

 

[LC2NLS6]

The "slider contraption" is the Safety Switch built into the Combination valve.

It doesn't block anything. It just moves over and turns on a dash light.

The front and rear brakes operate semi-independently because there's two high-pressure pistons (primary and secondary) in the master cylinder, not because there's a safety switch in the plumbing.

I've only read about those, and it was on roadkill garage episode once. Anyways, it was an idea on this dodgefordwilwooddanagmdiesel conversion. Has to be something simple, like blocked line or something.