Brakes dragging question

[rockyrivermark]

1970 C10
350 auto

So I'm still getting a dragging on my front disc brakes but it only happens when the engine is hot or I'm driving around a while. It's not severe but rotors are very hot to the touch after I go on a 15 min drive.
Finished the restoration this past June. Truck sat for a few years before I bought it this past year so I replace most of the braking system parts
Replace parts
New Front calipers
Rear wheel cylinders
Calipers brake hoses
Master cylinder
Adjusted master cylinder push rod
Master cylinder is not over filled
I've read that the brake fluid can get a vapor lock from the hot engine bay and the article suggested a performance brake fluid to help with this it's called Pentosin dot 4.
Entire system is bled and bled fine.
Thanks for any into or ideas.
Thanks
Mark

[rsgt]

My 72 was doing the exact same thing a couple weeks ago. I pulled the master cylinder loose, I left the lines hooked up. On the end of the master cylinder there was a rubber boot that looked like it could be getting pinched from the piston and maybe keeping the piston from coming all the way back. Not sure if that was the problem I removed the boot then checked to see if the rod coming out of the booster was too long holding the piston out, it seemed to be so I slid the rod out of the booster and carefully sanded about 1/16 off. I have not had any more trouble.

[Steeveedee]

Put a couple of washers between the master and booster. I suspect that you might have the pedal travel slightly off. It's an easy way to check. BTW, 1/16" at the master is a lot more at the pedal.

[rockyrivermark]

Originally Posted by Steeveedee

Put a couple of washers between the master and booster. I suspect that you might have the pedal travel slightly off. It's an easy way to check. BTW, 1/16" at the master is a lot more at the pedal.

Why would it only be a problem when engine hot?
I'll try it. I've pulled the master off with lines attached 100 times already. Adjusted thay rod a few times. That's definatly not too long
I just put the high temp resistant brake fluid it. Was gonna test it then it rained!
Thanks. Keep the ideas coming. It's driving me nuts!
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[rockyrivermark]

Originally Posted by rockyrivermark

Why would it only be a problem when engine hot?
I'll try it. I've pulled the master off with lines attached 100 times already. Adjusted thay rod a few times. That's definatly not too long
I just put the high temp resistant brake fluid it. Was gonna test it then it rained!
Thanks. Keep the ideas coming. It's driving me nuts!
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Tried the washer and adjust booster rod in more.
Went for a cruise. Came home and put thermo gun on rotors. 240 degrees
I know the pads run slightly on the rotors when driving but that temp seems excessive.
Any other ideas?
Mark
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[AussieinNC]

2 things immediately come to mind...

Master cylinder...drum brake m/c have a relief valve in the brake line port, under the tapered seat...it must be removed if you converted from drum to disk...the line valve is a small rubber plug and very small spring...remove the line going into the front brake circuit into the m/c and probe the port with a small piece of wire...if you feel the small valve, thread a small self tapping screw into the conical seat and pull the seat out...the valve and spring will fall out...replace the conical seat and bleed the circuit...and retest...

240 degrees is not all that hot for a disk temp after a run...these disks can sustain 250 to 300 easily...more important is make sure they are not binding...after a run, jack up each wheel and make sure they turn easily...

[mike16]

I'm betting on the Aussie in NC. I would also add that you made no mention of a distribution block or perportioning valve in your evaluation of your systems troubleshooting

You did not mention the perportioning valve, witch is not the same as a distribution manifold even though they look the same and are in the same location and often mistaken for each other

distribution manifold ( some vendors call it a valve too) is used in a drum drum set up. The perportioning valve is used on a disc/drum set up.

Big difference between the two and in what they do

a distribution valve maintains 15 lbs of pressure on the system so that the drum shoes dont retract completely. If the drums did retract completly then each time you hit the brakes you would get this very long pedal travel between initial application and the point where you feel the shoes contact the drum. it can be a bit scarry and that vital period of time may make the difference between stopping and hitting the obsticle in front of you. You would over compensate by riding the brakes and hitting them harder. The perportioning valve, in some vehicles aplies about 12-15 lbs of pressure to keep the brake shoes closer to the drum brake surface so this does not occure. it overcomes the pressure of the return springs that pull the shoes back towards the wheel cylinder Some brake pressure is maintines by valving within the master cylinder and others, within the distribution manifold/valve its self. Its important to properly adjust your brake system for this to work properly,

A perportioning valve does the same thing but only applies about 2 lbs of pressure on the front disc brake calipers, for the same reason, to maintain a small distance between the front disc brake pads and the discs them selves. very little pressure so as not to wear out the pads or disc's or over heat the disc when not in use. the 2 lbs pressure keeps the pads close to the disc, keeps the disc'c clean of water and debris.

a perportioning valve also maintains a differential pressure between rear drums brakes and front disc brakes.

If you have a drum drum set up on a disc brake vehicle it will apply about 15 lbs of pressure to the calipers and thusly to the pads and eventually to the discs, wich will produce the symptoms you describe. it wont make a noticable difference to the rear drum brakes but will constantly apply 15 lbs of pressure to your discs wich you may not feel at the brake pedal or notice as drag but it will wear out your pads and discs over a short period of time and will continually overheat your front disc brake set up causing brake fade after a short period of time,

how ever if you have a disc/drum perportioning valve set up and it is functioning properly them something else could be malfunctioning on the system.

[rockyrivermark]

Yes I posted a new post asking about proportioning Valve.
Here's a pic of the combination or proportioning valve.
I believe the valve was left on from the drum/drum sustem.
Keep in mind I did not do this conversion. The previous owner did.
The master cylinder i just bought new and was purchased for a '71 so it's definitely a disc/drum master.
My next step was to get a disc/drum proportioning valve.
I think they require a seperste brake line for each caliper right?
Seems like this may be the issue.

[rockyrivermark]

Pic!

[AussieinNC]

Drum/Drum masters were and still are available up to and including 72...after that C-10 were all disk front...(I believe)...

Tell me, the reservoirs on the master...is the bigger one to the front or rear? The smaller sized one is the rear brake circuit...

The brake lines for a proportioning valve truck are available...look up 73 on...the valve mounts under the front chassis cross rail...and includes the prop valve and the brake fail light switch...

You could also make your own lines...and mount the valve anywhere you like...

Each front caliper has its own dedicated line...the rears use one line to the rear rubber hose, then split on a t fitting on the rear end...

Have fun...

[rockyrivermark]

Originally Posted by AussieinNC

Drum/Drum masters were and still are available up to and including 72...after that C-10 were all disk front...(I believe)...

Tell me, the reservoirs on the master...is the bigger one to the front or rear? The smaller sized one is the rear brake circuit...

The brake lines for a proportioning valve truck are available...look up 73 on...the valve mounts under the front chassis cross rail...and includes the prop valve and the brake fail light switch...

You could also make your own lines...and mount the valve anywhere you like...

Each front caliper has its own dedicated line...the rears use one line to the rear rubber hose, then split on a t fitting on the rear end...

Have fun...

Back res is much smaller so I'm pretty sure it's a disc /drum master right?

[AussieinNC]

If the rear res is smaller, it is a disk/drum master cyl...

The rear port should be feeding the rear brakes,,,in other words, the lower pipe at rear of brake warning block should be feeding rear brakes...actually follow the pipe down and it should go under the front of the engine then follow the passenger side frame rail...(inside it) to the rear brakes...the rear port in the master should have the relief valve in place.

The front port should feed the front brakes and in your case, should split at a fitting on drivers side chassis rail.

The front port should not have a relief valve in the port fitting...if it does, it needs to come out...

[snipescastle2]

Residual pressure valves hold pressure to keep brake shoes out so you won't have to pump the brakes in order to stop in a panic.

[rockyrivermark]

Originally Posted by snipescastle2

Residual pressure valves hold pressure to keep brake shoes out so you won't have to pump the brakes in order to stop in a panic.

That valve is in the master cylinder right?
If so the master cylinder is new and disc/drum for sure.
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[67 chevelle]

Originally Posted by Steeveedee

Put a couple of washers between the master and booster. I suspect that you might have the pedal travel slightly off. It's an easy way to check. BTW, 1/16" at the master is a lot more at the pedal.

BINGO ,I have had to do this on every aftermarket brake system I have installed , took me a year to figure on a chevelle

[rockyrivermark]

Originally Posted by 67 chevelle

BINGO ,I have had to do this on every aftermarket brake system I have installed , took me a year to figure on a chevelle

Unfortunate that didn't work.
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[Steeveedee]

There is no such thing as a "residual pressure valve" in the master cylinder. Self adjusting brakes take up for wear on drum brakes, and brake fluid takes up the wear on the calipers. Springs in the master cylinder are meant to return the piston(s) to the starting position, and do not allow for a "hold" pressure in the system. Your mileage would take a pretty bad hit if your brakes were dragging.

[RichardJ]

>>drum brake m/c have a relief valve in the brake line port<<

>>a distribution valve maintains 15 lbs of pressure on the system so that the drum shoes dont retract<<

"Residual Pressure Valves" is what both of you are attempting to describe.
Residual Valves are seldom installed inside the MC any more. The rear drum cylinders uses a cup spreader that does the job of keeping the pistons tight against the shoes.

rockyrivers post #9 has a picture of a drum/drum type switch block that would not be used with a disc/drum setup.
The result of not using the correct "Combination Valve" or at minimum a "Proportioning Valve", would be that the rear brakes should be far to easy to Lock-up under medium to heavy braking.

If you think you might have an internal Residual valve in the MC, the front disc brakes should have noticeable drag at low speeds. Jack one or both front wheels off the ground. A 10# to 15# residual valve would prevent you from spinning the tire by hand. Of course the same would be true if the rod between the pedal arm and the booster didn't have any free play. Or at least until you deplete the reserve vacuum in the booster.
The after market sells a 2# residual valve for disc brakes, but should be used only when the MC is placed below the floor boards and lower the wheel cylinders.

When the disc caliper pressure is released, the piston will retract only .004 - .005". Spinning the wheel will result in the pads rubbing only slightly and occasional as you spin the wheel.

[RichardJ]

In response to Steeveedee's post, either residual valves or cup spreaders must be used to prevent air from entering past the cups when the pressure is released.

Even with cup spreaders, residual valves need to be added when the MC is below the floor. Gravity will pull the fluid down from the wheel cylinders and air past the cups.

[my67c20]

So do the simple thing first. Take off the wheels and compress the caliper, like you were removing the caliper. Then press the brakes. Does the caliper slide freely? Did you paint the slides? Did you apply some anti-seize to the bolts and slides? Can you spin the wheel after your foot is off the brakes? Did the caliper back off a touch? If not and you crack the line does it back off? I do not think that the vehicle being warm is related, unless you got the brake lines running next to the exhaust and are boiling the fluid. Are you bleeders at top or the bottom? Got any pictures? Lots of people here are good at seeing problems from user photos.

[Steeveedee]

Originally Posted by RichardJ

In response to Steeveedee's post, either residual valves or cup spreaders must be used to prevent air from entering past the cups when the pressure is released.

Even with cup spreaders, residual valves need to be added when the MC is below the floor. Gravity will pull the fluid down from the wheel cylinders and air past the cups.

We're not talking about '50s (and older) tech, here. What you are saying is true for those vehicles with the master cylinder mounted low. The springs and cups in the wheel cylinders do not hold the shoes out to avoid excessive travel, either (as was promulgated in a couple of posts about residual pressure valves, and I just want it to be clear- I'm not quibbling with you about it). They are, as you said, there to keep the cups expanded to avoid air infiltration.

[CastIron]

I didn't see it mentioned before, but have you checked the stop light switch? Just a little too tight can cause an intermittent problem. I've seen this dozens of times, and a lot of parts replaced trying to figure out the problem.

[RichardJ]

>>The springs and cups in the wheel cylinders do not hold the shoes out to ....<<

Not what I'm talking about.
The cup spreader is the little piece of metal the spring pushes against. After they eliminated the residual valves inside the MC, they took more care in designing the spreader so it put more, even pressure against the thin outer edge of the cup to get a better seal.

I think the older ones were there more to keep the spring from cutting into the rubber, more than anything else. Pressure from the residual valve was supposed to keep fluid pressure against the outer edge of the rubber cup.

[MoFormula]

Rockyriver, I just posted almost the very same problem in the suspension section. I recently changed my brake booster (1972 GMC 2wd LWB). You can move the truck by hand when it is cold. However after a short drive, wheels are hot, brakes are smoking, and you cannot move the truck by hand. Almost seems to retain pressure when driving that bleeds back off when you let it sit and cool overnight. Hoping you get an answer.

[67 chevelle]

After reading RichardJ response made me think maybe your antirattle clips are missing , just a guess . make sure they are lubed correctly also