I have an '01 Convertible with base (non-ABS) brakes. The left rear brake does not work. I thought it might need bleeding, so we tried that. Nothing comes out of the bleed screw when someone steps on the brake pedal. I then thought it might be the hose, and disconnected the hose at the steel line. Again, with pressure on the pedal, nothing came out of the steel line. The right front brake is working fine, so it's not the master cylinder.
To me, that means either the proportioning valve or the brake line is plugged.
Any thoughts on how to diagnose/repair this properly? Can I blow the line out with compressed air? Can I blow out the proportioning valve to clear it? Any likelihood that I could damage the valve by blowing on it backwards?What's the best way to do it?

Anybody ever encounter this before?

 

Break the line to the left rear at the proportioning valve and at the wheel. Blow the line to see. Probably not the problem, but doesn't require parts.

Break the supply lines to the proportioning valves to check fluid movement. (Verify master cylinder)

Replace proportioning valve.

 

Any hope of blowing the proportioning valve clear?

If I break the line between the valve and the master cylinder, what's the best way to keep the master cylinder from draining and requiring bleeding?

 

There is always hope, but also the possibility of damage. The brakes are the most important system on your car when it comes to safety.

As far as keeping the fluid in the master, find a couple of plugs or a twisted up rag corner (not paper towels) to stuff in the exposed pipe. (A clean rag or plug! Don't add debris to the system.) Make sure the master is full when you start and you likely won't have any problem with air. Be sure to bleed the lines afterward. It'll take forever. I suggest that you bleed the lines until you have bright shiny new fluid coming out of all wheels. FYI, most of the time, even if you get air into the master, you can get it to bleed out without having to remove and "bench bleed" it.

If you are not confident in your repair, or your ability to repair, this is one system to let the professionals handle.

 

Final chapter in this saga. I had my "trusted" mechanic flush the system (again) and replace the proportioning valve (for the second time). I suspect that when he did it last time he replaced the valve and then flushed the system, which probably flushed some crud from the master cylinder into the new valve. He also decided he needed to replace the left rear hose so we did that.
Long and short of it is that it is now working and I have brakes on all four wheels. I don't think I'll go back to that shop though. Every time I go there I have to check very carefully under the hood afterward for things left dangling and not hooked up right.