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Some proportioning valve questions after a rear drum conversion

1975.Imperial

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Hello all, I asked this over in the C-body forum but got no useful answers so hoping you guys can help.

For my Imperial, due to parts availability, I've had to change the factory disc/disc setup to a disc/drum setup (backwards from how most builds go, I know). The problem is that my front brakes now don't release, at all, the car sat untouched for a month and the brakes were still seized on. From what I've read, that's a sign of a tripped combination/proportioning valve from when the front lines drained while replacing the calipers. Could anyone confirm that's what it might be, I did some google searches yesterday and that's the extent of my knowledge on prop valves. That's lead me to my main question, should I replaced the factory disc/disc combination/proportioning valve with a disc/drum one or will my current one work just as well (after I reset it, of course)

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It's not clear what you changed, you put drum brakes on the front?
 
Converted to factory styled drums on the rear, was originally discs on all 4 corners. Also replaced everything except the Master, Booster, Proportioning valve, and hardlines with new/reman stuff
 
Did everything work before the swap? You said things were stuck. You can have a bad master cylinder, stuck caliper up front. Would need to see pic of valve you have. Mopar used a few different types. But in general I do think you would want to use the set up Mopar did for Disc/Drum.

Proportioning valve will limit pressure to rear drums since less is needed (self applying). But that should not be the cause of a stuck front disc. Are both stuck?
 
Did everything work before the swap? You said things were stuck. You can have a bad master cylinder, stuck caliper up front. Would need to see pic of valve you have. Mopar used a few different types. But in general I do think you would want to use the set up Mopar did for Disc/Drum.

Proportioning valve will limit pressure to rear drums since less is needed (self applying). But that should not be the cause of a stuck front disc. Are both stuck?
The fronts worked before the swap, the rear drivers caliper piston was seized but the other side worked. Now both the fronts are stuck. I don't think that it's the master cylinder as when I was bleeding the brakes there was a decent amount of suction made by both reservoirs. I'm thinking that the combination valve closed the front lines, which is what's causing the sticking. I'm not quite sure how to reset it or if I'll need a different one for the updated setup
 
The fronts worked before the swap, the rear drivers caliper piston was seized but the other side worked. Now both the fronts are stuck. I don't think that it's the master cylinder as when I was bleeding the brakes there was a decent amount of suction made by both reservoirs. I'm thinking that the combination valve closed the front lines, which is what's causing the sticking. I'm not quite sure how to reset it or if I'll need a different one for the updated setup
If you crack the fitting from the front lines into the proportional valve and the caliper releases then you'll know.
 
You replaced the master or you didn't replace the master? Sounds like there's a residual valve in the front master port that shouldn't be there...
 
Drums have residual valve. Disc would not. Regardless, the swapping to drums in the rear should not have an effect on front disc sticking. Releasing pressure at disc and if it releases would indicate calipers are good. You still do not know why they are sticking. Combination valve issue, or Master Cylinder. You can have a piston sticking in the MC.
 
Another tip... if the proportioning valve is stuck one direction, it turns on your "brake" light on the dash. I'm pretty sure you'll need the correct master cylinder and proportioning valve.
It may be too late.... but what part wasn't available for the factory disc/disc? Calipers can be rebuilt
 
We need some definitions here. The emergency lite is when one side has a leak and a differential piston moves to make contact with arm to ground a warning light. In the early cars that is just part of the distribution block. The Proportioning valve early, was a separate valve on the rear drum line. This would reduce the pressure to the drums after a set point since drums need a lower pressure than disc calipers. A Metering or hold off valve was also a separate valve for the front that would delay application of pressure to the front disc to ensure you had drums making contact before the disc activated. KH system used this, early bendix did not. This prevents tail spinning the car especially on slippery surface. Later starting in 71ish they were combined into one assembly.

So something in the front system is not releasing pressure. Stuck MC piston, possible something in the combination valve front section, or the calipers.
 
Any kind of mineral oil *inadvertently * put in the master cylinder will produce these types of outcomes as well.
Just did one last week. Of course there was no owning up to it.
 
Another tip... if the proportioning valve is stuck one direction, it turns on your "brake" light on the dash. I'm pretty sure you'll need the correct master cylinder and proportioning valve.
It may be too late.... but what part wasn't available for the factory disc/disc? Calipers can be rebuilt
The rotors and parking brake parts aren't reproduced by anyone. The calipers are actually the same ones as the front ones for a Volare so they're still available. The brake light was on for a while but I think it burnt out. So there are differences between the two setups, I'll start trying to source them. Thank you!
 
We need some definitions here. The emergency lite is when one side has a leak and a differential piston moves to make contact with arm to ground a warning light. In the early cars that is just part of the distribution block. The Proportioning valve early, was a separate valve on the rear drum line. This would reduce the pressure to the drums after a set point since drums need a lower pressure than disc calipers. A Metering or hold off valve was also a separate valve for the front that would delay application of pressure to the front disc to ensure you had drums making contact before the disc activated. KH system used this, early bendix did not. This prevents tail spinning the car especially on slippery surface. Later starting in 71ish they were combined into one assembly.

So something in the front system is not releasing pressure. Stuck MC piston, possible something in the combination valve front section, or the calipers.
Thank you for the history, I love learning that sort of stuff.

After the responses from this thread, it seems like the master and combination valve are different between the disc/disc and disc/drum setup so I'll have to swap them out anyways, meaning no matter what's caused the problem it should be fixed.

Because this problem only started after the conversion, and because it was making suction while bleeding, I don't think it's the master. The calipers are new/reman, and the old ones stuck too, so I'm assuming that they're not the problem. From learning more about how the differential piston moves, I'm pretty sure that's my problem. Thank you for your help
 
Once you bleed you move junk through the system. I am not seeing the warning light piston as the issue, the high pressure on front side would push it towards rear. That leaves the ability to release front pressure. So how are they stuck. I don't disagree with REM about the wrong fluid either. I lived through that once.
 
Once you bleed you move junk through the system. I am not seeing the warning light piston as the issue, the high pressure on front side would push it towards rear. That leaves the ability to release front pressure. So how are they stuck. I don't disagree with REM about the wrong fluid either. I lived through that once.
Exactly, as with the 2500 06 chevy I had as an example. Buddy did the rear calipers( younger kid) they drove it and sooner or later the brakes are locking up. Long story short replace the m/c end and flush the best I can.
But watched the **** go through a clear hose

I have a container and I let the **** separate.
I'll get a pic lol.
Anyway, the first thing you do is Crack the line at the master cylinder. If that doesn't release the brakes, Crack the bleeder. That will tell you if it's the m/c, distribution block, or calipers.
 
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