No rear brakes

east302 said:

but when you look at the service procedure for replacing the valve, it says that the metering rod has to be held in during bleeding.

You must be registered for see images attach

I'm guessing that holding this pin down prevents flow into the front lines by keeping the metering valve closed, increasing pressure and allowing the differential piston to re-center and allow fluid into the rear lines.

Click to expand...

Two mistakes here.

1. The metering valve is intended to block fluid flow to the FRONT brakes. It has nothing do do with the rear brakes. Holding the pin in is to allow--not prevent--fluid to flow to the front. The only time this is required is when PRESSURE-BLEEDING the system; and only then when using moderate pressure. Low pressure isn't enough to close the valve, therefore there's no need to dick with the pin. High pressure is enough to pop the valve off it's seat, so there's no need to dick with the pin. But moderate pressure will close the metering valve which blocks fluid flow to the front brakes, so the pin has to be moved manually to get the fronts to bleed properly.

2. The differential piston doesn't block fluid flow. It activates the safety switch, but doesn't seal-off any passages. Most differential pistons are self-centering on GM. I've had a couple of Fords where they had to be manually reset.

is any fluid suppose to come out of the pin if it is pressed while pressing or depressing pedal?

No.

Schurkey said:

Two mistakes here.

1. The metering valve is intended to block fluid flow to the FRONT brakes. It has nothing do do with the rear brakes. Holding the pin in is to allow--not prevent--fluid to flow to the front. The only time this is required is when PRESSURE-BLEEDING the system; and only then when using moderate pressure. Low pressure isn't enough to close the valve, therefore there's no need to dick with the pin. High pressure is enough to pop the valve off it's seat, so there's no need to dick with the pin. But moderate pressure will close the metering valve which blocks fluid flow to the front brakes, so the pin has to be moved manually to get the fronts to bleed properly.

2. The differential piston doesn't block fluid flow. It activates the safety switch, but doesn't seal-off any passages. Most differential pistons are self-centering on GM. I've had a couple of Fords where they had to be manually reset.

Click to expand...

Im having a hard time getting fluid to the rear brakes. I have a new rubber hose in the back and no air in the fronts. I took out both rear bleeders and cleaned them in case they had a blockage. No fluid came out of the open holes. The backs of the bleeders were wet but not soaked. Only information I can find is saying the proportion valve is blocked like a 70s pickup that cuts off the rear or fronts when you lose brakes. However all information I can find says the proportion valve is in the Kelsey Hayes abs module. I would appreciate any direction to go in search of fluid. I did even try to crack the lines at the abs module and tried to bleed them from that location. I have fluid going from the master to the rear line to the abs module but not out of the abs module to the rear brakes.

Schurkey said:

No.

Click to expand...

What if it does?

2500longbed said:

Only information I can find is saying the proportion valve is blocked like a 70s pickup that cuts off the rear or fronts when you lose brakes.

Click to expand...

NOT the "proportion valve".

That would be the Safety Switch. And unless the BRAKE light is lit on the instrument cluster, it ain't that.

2500longbed said:

However all information I can find says the proportion valve is in the Kelsey Hayes abs module.

Click to expand...

Not "in" the ABS. The combination valve is bolted to the rear of it.

2500longbed said:

I would appreciate any direction to go in search of fluid. I did even try to crack the lines at the abs module and tried to bleed them from that location. I have fluid going from the master to the rear line to the abs module but not out of the abs module to the rear brakes.

Click to expand...

ABS has multiple valves. Three of them close to block fluid flow, (Isolation Valves) three of them open to allow fluid to blow-off into an accumulator (Dump Valves).

MAYBE you have a perpetually-closed rear Isolation Valve that should be normally-open.

Connect a scan tool, chatter the ABS valves by performing an auto-bleed, see if the valve(s) unstick.

Schurkey said:

NOT the "proportion valve".

That would be the Safety Switch. And unless the BRAKE light is lit on the instrument cluster, it ain't that.


Not "in" the ABS. The combination valve is bolted to the rear of it.


ABS has multiple valves. Three of them close to block fluid flow, (Isolation Valves) three of them open to allow fluid to blow-off into an accumulator (Dump Valves).

MAYBE you have a perpetually-closed rear Isolation Valve that should be normally-open.

Connect a scan tool, chatter the ABS valves by performing an auto-bleed, see if the valve(s) unstick.

Click to expand...

Unfortunately I don't have access to a bi directional scanner. I only have a code reader and eraser. If I had a scanner capable I wouldn't need assistance as it would inform me of whats going on. I can't imagine a gm product that can't be repaired without a scan tool. This isn't a land Rover or McLaren.

2500longbed said:

Unfortunately I don't have access to a bi directional scanner. I only have a code reader and eraser. If I had a scanner capable I wouldn't need assistance as it would inform me of whats going on. I can't imagine a gm product that can't be repaired without a scan tool. This isn't a land Rover or McLaren.

Click to expand...

Trying to repair a computer-controlled vehicle without being able to properly communicate with the computer is two steps away from madness.

"Codes" are the BEGINNING of computer diagnostics. The REAL diagnostic power is in the data stream, and the bi-directional control. As such, "code readers" are not all that helpful.

GM products tend to have had the most-sophisticated computers of any popular make in any given model year.

I have heard talk of chattering the ABS valves by jumping two terminals on the diagnostic connector...but I've never done that myself. Maybe someone else will elaborate.

Schurkey said:

Trying to repair a computer-controlled vehicle without being able to properly communicate with the computer is two steps away from madness.

"Codes" are the BEGINNING of computer diagnostics. The REAL diagnostic power is in the data stream, and the bi-directional control. As such, "code readers" are not all that helpful.

GM products tend to have had the most-sophisticated computers of any popular make in any given model year.

I have heard talk of chattering the ABS valves by jumping two terminals on the diagnostic connector...but I've never done that myself. Maybe someone else will elaborate.

Click to expand...

Sure trying to diagnose a computer without a scanner tool is madness. However I'm trying to get fluid to the rear brakes not diagnose a timing issue or running problem. It's brakes. The abs only reduces flow to the rear in a uneven braking surface or low traction but should not have any limiting to the fluid flow. This isn't a computer issue it's a lack of a good diagram or service procedure. Just because the dealership did it with a scanner doesn't mean I need a tech 2 or comparable scanner to fix anything on this truck. I did the intake gaskets and timed it without any of that without issue. I'm cleaning up previous owner issues so I don't even know when the last time it had fluid. Rabbit holes into madness is exactly what fourms like this are for. Not all of us have access to expensive computer scanners and don't rely on them to find problems.