Level sensing prop valve on 1-tons

[Mastiff]

I'm having a heck of a time getting my brakes to have a firm pedal with low travel since I dropped in the CUCV axles. I've bled four big bottles of fluid through it with a pressure bleeder. The brakes work, but there is a lot of travel. One thing I was wondering about is the fact that the CUCVs, and I think all K30's, had the extra proportioning valve in the rear that sensed the load and adjusted accordingly. I assume that full flow went to the rear brakes only under max load? As it stands now, I just hooked up my old flex line to the new axle. I wonder if this is a contributor to what's going on. Any thoughts?

Also, is it possible for the regular combination valve to be in a state where it needs to be reset or something? I see a little rubber button on the valve, but I don't know what the normal state of it is. It's poking out a bit at the moment.

 

[dyeager535]

Theoretically the combination valve is *supposed* to reset itself, but I know some have to reset it manually. Is your brake light working properly when you bleed it/when done? Again theoretically, when you crack a bleeder with the brakes depressed, the light should come on if key is on. If it goes off after closing the bleeder and letting the pedal up, it's resetting ok.

Did you change the master? If it's at all spongy after thorough bleeding, the master is potentially garbage.

1/2 to 1 ton swap? I would suspect more fluid needs moved to the rear than the master will allow (hence different masters with disk brake rears) but I've not dealt with the 1 ton rears.

I need to check the parts manual, I wonder if the combination valves are different. I've never seen that mentioned before.

Forgot the load sensing prop. valve...pretty sure no one keeps those things lol. Universally discarded. It must work to some degree, GM wouldn't have installed them, but I can't recall anyone saying they absolutely loved it.

 

[Mastiff]

I replaced the master with a 1-ton master. Someone mentioned replacing the prop valve with a 1-ton model, but as far as I can tell they aren't available for sale anymore except from salvage. I don't want to go down that road unless there's more consensus that it's needed.

I have never seen the brake warning light, but the key was never on during bleeding either.

I wonder is there any way I could have messed up the D60 caliper install such that they aren't adjusting properly? My symptoms might be consistent with too much piston travel before there's any resistance. It didn't seem like there were a lot of options, but I was starting from scratch with nothing to copy. The shoes are definitely adjusted properly. If anything they have a little too much drag.

 

[Fordum]

This is probably wrong, I'm not familiar enough with the various GM products to be sure.
But its only a matter of seconds to check.
Some calipers on some axles can be installed on the wrong side. They fit and work just fine, but the bleeder is on the bottom instead of the top.

When that happens, its pretty much impossible to get the air out.
Just glance under yours, and if the bleeders are on the top, then disregard.

 

[4xcrazy]

My Burb had much more pedal travel and a little more soggy feeling when I switched over to rear discs. The brakes work great, just a bit softer than I was used to. Never done much with it, because they work.

 

[Mastiff]

This is probably wrong, I'm not familiar enough with the various GM products to be sure.
But its only a matter of seconds to check.
Some calipers on some axles can be installed on the wrong side. They fit and work just fine, but the bleeder is on the bottom instead of the top.

When that happens, its pretty much impossible to get the air out.
Just glance under yours, and if the bleeders are on the top, then disregard.

Yeah, I've seen this mentioned. Unless I'm confused, it'd be pretty tough to get it put together that way. Mine has the flex line entering at the top and the bleeder facing straight back but very near the top. If it was on upside down, wouldn't the flex have to enter on the bottom?

 

[-stew-]

That load sensing valve isn't the issue. I took the one off my truck and the brakes are fine with no change in pedal feel or travel.

 

[dyeager535]

I'm going to have to look up the combination valves in the GM manual.

If using all stock 1 ton components (booster, calipers/wheel cylinders) or components that are identical, obviously pedal feel should be the same. Not identical situation/application, but I knew I had seen something about different combination valves in GM's: http://www.gofasst.us/brake_proportioning1.htm If the cylinder size is different between the 1/2 and 1 ton, and the combination valve was different, I would think it safe to assume volume was the reason for the difference. Clearly understanding the difference(s) might help everyone, including the disk rear swap folks.

Are the drum shoes adjusted properly and on properly (if they've been messed with at all)? I found that a PO, with some work, had managed to put the brake shoes on wrong on my 14SF.

 

[wasted wages]

I've stated this before....

Pull the calipers off and wire them up so that the bleeder is pointing straight up....

block the pads or piston with a 3/4 piece of plywood...

Step on the brake pedal to push the piston or pads out against the plywood.

Crack the bleeder screw and let it drain into a bucket.

Keep topping off the M/C with fluid and just let the caliper gravity bleed.

Do all four wheels this way and it should work fine....I work by myself and don't usually have a helper around to pump the pedal for me.. so I know this works..You can also use a vacuum or pressure type bleeder,,,but the key is to have the bleeder pointing up to let all the bubbles escape from the caliper bore.

 

[Mastiff]

Do all four wheels this way and it should work fine....I work by myself and don't usually have a helper around to pump the pedal for me.. so I know this works..You can also use a vacuum or pressure type bleeder,,,but the key is to have the bleeder pointing up to let all the bubbles escape from the caliper bore.

Worth a shot I guess. I saw you suggest this before and haven't done it because the calipers are a pain to get on and off. I do happen to have a ton of caster at the moment, which might not be helping... The guys at the parts place are going to look at me funny as I buy my 5th and 6th big bottle of brake fluid.

 

[wasted wages]

At least you will have eliminated the possibility of air bubbles in the lines and calipers.

If the lines and calipers are full of brake fluid, with no air bubbles...
then you can start blaming other parts or looking at other issues.

 

[dyeager535]

Yes, there are about 20 combination valves used between 1985-1989 in "our" body styles. I didn't spend a ton of time looking, but appears that the 1/2 and 3/4 ton trucks with "standard" brakes all got the same valve, but one tons most definitely had a different valve.

Additionally, at least on the plastic reservoir brake master cylinder (didn't check the other styles), two different secondary pistons were used depending on brake RPO.

 

[Mastiff]

Yes, there are about 20 combination valves used between 1985-1989 in "our" body styles. I didn't spend a ton of time looking, but appears that the 1/2 and 3/4 ton trucks with "standard" brakes all got the same valve, but one tons most definitely had a different valve.

Additionally, at least on the plastic reservoir brake master cylinder (didn't check the other styles), two different secondary pistons were used depending on brake RPO.

Interesting. So the question is whether it makes a significant difference. And were the K30 ones designed specifically to work with the load sensing valve in the rear.

As far as the symptom of too much pedal travel goes, can the prop valve really affect that anyway? All they can do is change the size of passages, the total volume of the system is essentially the same with or without them, so it seems like the bottoming out point of the pedal would be the same.

It keeps coming back to air in the system I think. Even though it seems like I've bled the hell out of the thing.

The service manual talks about using some special tool to hold the prop valve while bleeding. Anyone know about this? I guess it makes sense that the valve would fight against dumping fluid out the bleeder at a high rate. That's part of its job. But then nobody seems to talk about that when discussing bleeding procedures.

EDIT: http://www.ebay.com/itm/Combination...leeder-Tool-Kit-Brakes-For-Sale-/160856168243

 

[82355]

I know I took my valve out of the system on my 1982 K30 3+3 because it kept locking the rear brakes up with the lift causing the valve to be out of wack. The brakes work just fine now.

Martin

 

[dyeager535]

I've never heard of anyone using that tool...at worst people manually reset it. I've bled mine a couple of times (same valve) never had to mess with the combination valve.

I think the different proportioning/masters is more of a curiosity in this case than a cause. Air is most likely culprit, but with a different secondary piston potentially pushing more fluid, I would think that COULD be a possible issue, or at least contributes.

You are correct, there are a TON of variables that could affect the need to proportion differently...load sensing valve, all the different brake RPO's, the body style and axles/t-case used (weight/distribution) and so on. I only looked at 5 years worth...who knows what the situation looked like starting back in 1973. I noticed it looked like the hydroboost setups had the valve much closer to the master assembly, and were definitely different PN's, FWIW.

With all the problems folks have when going to disk, it only makes sense (IMO) that if you go with bigger brakes in the rear, even drum, you are at least in a small manner, affecting how the system works with what parts you have and what you've added.