Hydro-boost Brake System Problem...

[DoggyDiesel]

Hello!

I now have to vehicles that have the same symptoms regarding braking. One of these is a 1982 Cadillac with a 6.2 diesel in it, and the other is a 1982 C20 with a 6.2 diesel. Both have Hydro-Boost brake systems. The brake systems are intact on both vehicles. (e.i. lines are not rusted through, hydro-boost is not leaking, master reservoirs have fluid in them...) I have bled the brakes on each. When I start the vehicles up and apply the brake, the pedal goes all the way to the floor. While driving, there is approximately 1/2 inch (if that) of poor brake action.
My thoughts have been the master cylinder could be bad, I maybe need to reset or bleed the proportioning valve, or ????
The brake light is on in the dash as well on both.
Any input would be great as I don't want to rush into replacing a bunch of parts that I wouldn't have had to in the first place.
Thanks!

 

[Snoots]

If your BRAKE light is on, you have air in the proportioning valve.
1. Did you have a helper when you first bled the system?
2. Did you start with the right rear?
3. Did some clown shake up your brake fluid bottle before you got it?
Those dam itty bitty bubbles add up.
Then, you'll need to press the RESET button on the PV.
No need to replace parts, yet. If they are the original MC's, they would be my next guess as a problem.

 

[Scott91370]

Did you bench bleed it before installing?

I'm not 100% sure that's needed on this system, but if you strarted dry it may have air trapped.

 

[DoggyDiesel]

1. Did you have a helper when you first bled the system?

Yes, I did have help when I bled them.

2. Did you start with the right rear?

I did indeed start with the wheel end furthest away from the MC.

Then, you'll need to press the RESET button on the PV.

I thought perhaps knowing there is a bleed screw on the PV, I can bleed the air out of it there? Is that how one resets the valve? Again, this Hydro-Boost is a bit of a different setup for me.

 

[fast 99]

PV may be stuck off center but there is no bleeding them. They bleed with system bleed. What was changed? Brake systems don't generally get low pedals by themselves.