Flushing brake system - what's wrong with my rear circuits/proportioning valves?

[JFerguson]

Related thread here.

I've been having problems with my brakes (mushy, fade, like air in system) and am currently overhauling them and replacing some bad lines. My car (1991 Oldsmobile 98) has four individual circuits. My master cylinder feeds two lines into my ABS hydraulic unit (aka Pressure Modulator Valve (PMV)) and this unit then feeds four lines to the four wheels. I have the ABS hydraulic unit off the car and here are the lines:



I took an air compressor, and for each line, opened up the corresponding bleeder and blew all old fluid out.

Here's the issue, though. My right rear circuit cleared with no problems. My left rear circuit did not; I could not blow through this line, all the fluid just came right back out (and sprayed somewhat, from the pressure) where I was blowing into it (in the above picture).

The other thing I noticed was that when I was first removing the rear wheels on this job, I had the back end off the ground, and my son was applying the brakes. This left rear wheel would grab, but then it would spin freely a little when I was loosening the lug nuts.

Here's a picture of the proportioning valves:



The old one in the foreground is the right rear circuit, which I was able to flush through with the compressed air. The new one in the background (replaced in '02, according to my service records) is the left rear circuit, which I could not flush through.

I understand what proportioning valves are supposed to do -- I guess I'm wondering if one of these valves is possibly not functioning properly, and if so, which one?

Thanks...

[abaird]

I would replace it since you are overhauling the brakes anyway. Or at least disconnect it on both ends and see if you can force air through the lines without the valve. Remember that brake lines are pretty thin and just a little kink in the line could pinch it off.

[TheSilentChamber]

That loks more like a residual pressure valve than a prop valve, do like said above and take it out and blow though the lines and see if it is the problem, if so replace it.