Thanks again for the mountain of replies you guys. Knowing that there is a mountain of knowledge here to help me is literally all that's keeping me sane with this car right now.

@ Pete, noted, but I still think something is amiss. While it's true that a "healthy" pedal should have some give to it, it should feel like one hits a 'no-go' point during the pedal's travel, not a gradually increasing stiffness (i.e. sponginess), which is what I'm getting now. A few of these lines still look a tad ratty, so I'm going to pull a few more tonight and do my diagnostic procedure over again and reconnect things in a different order. I've clearly missed something. As I mentioned before, with the master totally blocked off, the pedal is tight and stiff as can be (as it should be,) and, in theory, this is how the system should feel when it's properly bled and all of that.

@ carrott Don't worry, I don't plan to test it on the road just yet! Also, I've heard about this plate -- I was having symptoms before I replaced the master, is it possible that this thing can fall out without disassembly, or even break? I don't have my FSM right in front of me, but can you maybe link me to a bit more information about this part? Given that the pedal feels great when the master is blocked off, I'm inclined to think this isn't my issue, but at this point I will investigate anything somebody suggests to me.

@ Travel'n Man Thanks! That's the order I came up with as well, and that's what I'll do for sure when the girlfriend is back at the house this weekend.

@ John Coffey Okay, thanks for clearing that up. I'll pay more attention to how/when I crack open the screws the next round of bleeding. I can see how, after thinking about it, why that would make the most sense.

Thanks again for your replies everyone. I'm frustrated to no end, but glad you guys are here for me. :x

EDIT: I've also seen in other posts folks saying it's important to pull apart and clean out the proportioning valve?? Thoughts on this? Is there a procedure for that?

Edited May 9, 201311 yr by KDMatt

Having cleaned out this past winter I couldn't believe how much junk was in there. Once you have it in hand there is a small spring clip holding the valve in place, there will also be a lot of grit built up around the base of the valve, needle nose pliers will do the trick, clean it out and reassemble. Once every 40yrs is the maintenance schedule I guess.

Okay guys, quick update.

I checked between the booster and master cylinder and I didn't see anything that looks like it's fallen out of place or is disconnected.

Also, I removed the proportioning valve and took it apart to clean today. To my surprise it was actually reasonably clean.

Here's the thing though... in continuing with my diagnostic procedure, I connected the (freshly cleaned) proportioning valve and plugged off its ends, and suddenly the squishiness in the brake pedal has returned. I've tightened the ends on it as tight as they will go (as there was initially some leaking on the top one, which has now been rectified with some snug wrenching...) Granted there might be a tiny bit of air trapped in there now that I've cracked it open and haven't really bled the system yet, but I'm wondering if the internal seals on these things can go bad enough to actually draw air into the system inadvertently?

I'm sorry if I'm not making a ton of sense -- it's been a late night, but the real question here is: can the proportioning valve/assembly "go bad?"

Thanks,

- Matt

I say its the brake master. I had very similar problems like you and I finally pulled apart the master and found a partially

collapsed rubber seal.

Hmm... I suppose that's possible?

Thing is, I had this problem with both my old master, and now with my new replacement one.

When I plug off the outlets from the master, the squishiness goes away, to the extent that the pedal will barely move (as you might imagine it would) which leads me to believe that the master is in pretty good working order.

I don't notice any pedal sponginess returning until I get the proportioning valve hooked back up, so I'm really not sure what that's all about.

When I bought my car the PO had replaced the vacuum source line to the power boost and had omitted the servo check valve resulting in no boost. Six months later the boost unit failed when its diaphragm ruptured. In both cases braking action was weak to nil but the pedal was stiffer than when the boost was working properly.

.

Are you sure when you put in the new check valve its going in the right direction?

If its going the wrong direction you wont be getting any vacuum to the booster

Also, if someone before you had a 15/16" master they may have changed that little adjustable

rod that goes between the master cylinder and the booster unit. Do you have a 7/8" in there now?

Yes, the check valve for the booster should be correct, as I replaced the booster several years ago and have had no issues with with anything brake related up until now. In fact, even now, the booster is still giving me more braking power when the engine is running as it's supposed to.

I'm having a different problem than user "djwarner" as my pedal is not stiff, but spongey.

I'll double check the rod for the master, but as I mentioned before, I was having issues before I even touched the master cylinder, so I don't think this is my problem either.

Still curious to hear what people think about the proportioning valve.

Yes, the check valve for the booster should be correct, as I replaced the booster several years ago and have had no issues with with anything brake related up until now. In fact, even now, the booster is still giving me more braking power when the engine is running as it's supposed to.

I'm having a different problem than user "djwarner" as my pedal is not stiff, but spongey.

I'll double check the rod for the master, but as I mentioned before, I was having issues before I even touched the master cylinder, so I don't think this is my problem either.

Still curious to hear what people think about the proportioning valve.

The proportioning valve just redirects fluid away from the rears to prevent them from locking up.

Definition:

The proportioning valve modulates the pressure to the rear brakes. The modulation is necessary to minimize rear wheel lock up found in heavy braking and to compensate for the differences in braking conditions in front disc / rear drum systems. As pressure is applied to the system full pressure is allowed to the rear drums up to a certain point. Beyond that point the pressure to the rear is reduced preventing rear brake lock up.

Symptoms of a bad proportioning valve:

If your prop valve has gone bad your rear wheels will lock up easily, especially on wet surfaces.

Symptoms of a bad proportioning valve:

If your prop valve has gone bad your rear wheels will lock up easily, especially on wet surfaces.

FWIW, it's the other way around. Your rears will tend to lock before the fronts in the dry more so than the wet. There is less weight transfer in the wet (less grip), thus the rear axle is more loaded and less likely to lock prematurely.

Hmm... the last time I tried, I couldn't get my wheels to lock at all...

It seems like the further down the line I go (in terms of plugging fittings and testing for pressure), the less power I have.

Could I have had two bad master cylinders in a row? I'm still very puzzled about the behavior of these brakes.

Have you checked your flexible lines? The ones that run from the body to the suspension. You might have one about to blow. Expanding when pressure's applied, contracting when released. Might need help to watch while the pedal is pressed.

Have you checked your flexible lines? The ones that run from the body to the suspension. You might have one about to blow. Expanding when pressure's applied, contracting when released. Might need help to watch while the pedal is pressed.

The Flex lines have all been replaced. The rears were done last year, and the fronts were done two years ago. Thanks for the suggestion though.