The Stovebolt Site Forums The Shop Area Driveline Brakes drag after 10 minutes.

Right now I’m so frustrated I can’t think straight.
I just finished installing a new dual brake master cylinder and booster in my 1965C 10 I/2 ton. I was previously using a 1995 Blazer MC and brake booster. The brakes worked fine but I noticed that after 10 or 15 minutes of driving, the brakes would begin to drag. You could let off of the brake pedal on a slight incline and the truck would not coast backwards.
This new master cylinder and brake booster are doing the EXACT SAME THING. My 63 C10 has the exact same MC and brake booster with front disc and rear drums and I have absolutely no problems there.
I am certain that the brake pushrod is allowing the master cylinder plunger to completely return to the home position.
My next thought is that the pistons in the front disc brake calipers are not moving correctly. If this is true, then they do not pull the brake pads completely off the rotor as they should.
Anyone have thoughts on this matter?

Once the brakes start to drag, stop the truck, chock a wheel front and rear, and loosen the bleeder screws on the calipers. It's possible you're trapping fluid in one or both calipers due to a flex line collapsing inside. If you get a spurt of fluid and the brake drag releases, replace the flex line on the offending side, or both of them if that hasn't been done recently. I've seen several examples of that problem over several years, one of which got so bad a transmission failed because the car owner was too stubborn to stop and call for a tow truck, and kept on driving with the brakes dragging.
Jerry

Thanks for your comment Jerry.
So if I understand you correctly, when pressure in the brake lines goes to zero (when I let off of the brake pedal), the flexible brake hose may be collapsing and preventing the fluid behind the caliper piston from going to zero. Right?

Yep, I've seen it happen frequently. If the caliper doesn't release when the bleeder is opened and the wheel still drags (jack up the wheel to check that) then you have a sticky piston in the caliper.
Jerry

Someone PLEASE come up with an answer for this. My 37 Buick worked fine for 30 years with a drum-drum dual master cylinder and power booster. Got caught in a dicey situation in Kansas City during a monsoon where the drums were under water so decided to put discs on the front. Put the disc-drum dual mc on and even the hubcaps are hot to the touch after ten miles. Decided the supposed to be 10# drum and 2# disc residual pressure seats in the mc were wrong so to test I put the old mc back on after drilling through the flare seats to eliminate ANY residual pressure. Same thing so replaced new hoses that came with the disc kit with two more new hoses. Same thing. Took some 3/16 steel line with a double loop and replaced rubber line for a (very) temporary test. Same thing. The link from pedal to booster is floppy loose when spring retracts pedal and have shortened the rod from booster to mc till the pedal has gone from near full height to 1.5 iches off the floor. When the car stops as soon as you leave up on the gas I jump out with a 10mm wrench and crack the bleeders which makes it completely free. Summary: NO rubber lines, NO residual pressure valves, rods NOT touching the booster or mc, what's happening? Oh yeah, used spare mc cap with two large vent holes in it for testing. Drove into shop while nearly sliding front wheels and cracked left bleeder--wheel spun free but right side was still nearly locked.

Evan, have you tried swapping to a different caliper on the brake that drags? What's supposed to happen when the pressure releases after a brake application is the square O ring that seals the fluid is also supposed to retract the piston a few thousandths when it returns to "square" after being distorted slightly when the brakes are applied. At times, I have had to do some serious honing of the caliper bore so the piston is a sloppy loose fit. Otherwise, the piston can't retract enough to keep the pads from dragging on the disc. Back when Ford was using plastic caliper pistons, it wasn't a question of IF, but WHEN the brakes would drag. I can't count the number of chrome plated metal caliper pistons I swapped out on early 1980's Ford brakes!
Jerry

Along with the already mentioned fixes.........debris in the master cylinder that is blocking the return port with cause brake drag.

Picture is dual master so it shows two circuits.

UPDATE:
I just drove the truck for 10 minutes and the brakes have gotten really tight. I drove into the shop and put it up on the lift. The rear wheels turn freely with the transmission in neutral. No drag.
The front wheels are so tight I cannot turn them by hand.
So one possibility is simultaneous collapse in both front brake hoses.
The second possibility is that the caliper pistons are not moving smoothly.

Replace all in front, calipers and hoses. Or unless theres a fluid issue of sorts from the master cylinder?

Did you try opening the bleeder screws to see if the drag goes away? That's the best way to tell the difference in restricted hoses or sticky calipers.
Jerry

There was a squirt of fluid from the bleeder valve and I can now turn the rotor by hand, BUT . . . . there is still some drag on the rotor. So I’m not 100% sure I know which it is, or both.

I did not divulge every piece of information at the beginning of this thread. I did not want to complicate the issue since there were so many factors involved.
Here is what we now know to be true.
1. The front brakes get tighter and tighter. The rear brakes are fine.
2. When I open the front caliper bleeder valve, there is a squirt of fluid. This indicates that pressure is being retained in the front brake lines somewhere.
3. When I opened the opposite side front caliper bleeder valve, there was no indication of retained pressure. Both rotors now turn freely.

This would seem to indicate that the brake hoses are not collapsed. If both brake hoses were collapsing, I would think there would be a squirt of fluid from each caliper. Yet, both front wheels were too tight to turn by hand until I released pressure from one of the bleeder valves. Once I released pressure from one of the bleeder valves, both rotors could be turned by hand.

Now please don’t be mad at me for withholding the following information. (Or, you can be mad if you want, just don’t tell me).
I am using an adjustable brake bias on the rear brake line. I have it adjusted to the middle position. If this brake bias device is malfunctioning, perhaps it is not allowing the MC piston to return to the home position, even though the brake pedal push rod is loose and free. However, if that is the case, then I would think I would build pressure in the rear lines, since the two circuits are independent. Is it possible that the brake bias is hindering the MC piston from returning to the home position?

Hmmm. Drive it again. Get em stuck. Open the rear bleeder at the master and see what if any thing happens. If not open the front at the master. To have both stuck and free up after only opening 1 side indicates that it has to do with a common denominator, master cylinder is all i can suggest without being there and experiencing it for my self.

Expecting all the drag to go away is not realistic. The pads are going to remain in contact with the rotor. If opening one bleeder frees up both brakes, you're holding pressure in the front brake lines, probably all the way back to the master cylinder. Try loosening a line at the master cylinder and see if you get a pressure squirt there the nexct time the brakes drag.

I would be reluctant to use ANY sort of aftermarket pressure control device that the factory engineers didn't include in the original system- - - -chances are it was invented by some hotrodder who is going to solve all your problems by selling you something! P.T. Barnum would have been a good parts salesman!
Jerry

Are you using a proportional valve in the system? That failing has been my experince with having both front brakes gradualy locking up and staying locked up. If you have one there and have replaced everything but that,then give it a whirl.

Why would you ever start replacing parts with out finding the cause problem ?????

This could be as simple that the brake push rod is not returning completely or the five other things already mentioned,

Think it been already narrowed down to the master, proportioning valve and associated hardware.

UPDATE!
I eliminated the rear brake circuit brake bias. I joined the rear lines with a union nut. No more brake bias.

Took it for a drive. Brakes tighten up within 10 minutes. Same as before. At least we know not to blame the brake bias.

My next step is to disconnect the front hard lines from the hoses and check flow back towards the MC. There is only one T junction block in the front circuit.

FYI: I’m using 1/4” hard line from the MC to the brake hoses with a 1/4” to 3/16” adapter nut at the front brake hoses because the hoses only accept that size thread. Most Chevy front disc brake hoses use a 3/16” hard line nut.

I know you said the brake push rod was properly adjusted, but, just for funzies , back it off a couple of turns and try the test drive again. Don’t ask me how I know.🛠

Since a dual chamber master cylinder has a piston for each circuit, it's possible that one relief port is getting uncovered and the other one isn't. I can remember one incident where it took three master cylinders before I found one that didn't cause a set of front brakes to drag.
Jerry

“Been there, done that, got the T-shirt“ as they say. I will do as you ask. That was my first thought BEFORE I swapped out the old MC and booster for a new one. The reason I wanted a new MC and booster was so I could go back to my factory air intake and filter. My Blazer booster was too big and in the way. I’ve already done this same setup in my 63 and I love it.

As Jerry just mentioned, you could have a bad M/C, I too have had that issue.

Justhorsenround,
I backed the pushrod up 1/4” and went for a drive. About lap 3 around the block I began to panic because I sure I was gonna have to eat humble pie. But by lap 6 I began to feel the drag again. You had me sweating big time. The push rod is sloppy loose and the brakes are tight as a tick right now.
So I’m back to looking at a problem with either the MC or something in the front lines that restricts return flow to the MC.
I even ran the engine at idle in the driveway until it got hot under the hood. The exhaust downpipe was 250* but the brake hard lines were 150* at the level of the frame where they are closest to the downpipe. I pumped the pedal repeatedly but could not get it to tighten up like it does when driving.

Maybe I missed you saying, but are you using a 2# residual valve on the fronts? If so, maybe it is failing? Or, there is a 10# one there?

Dennis

Do you have a known good master cylinder? If not, buy one, it wont goto waste. If you eliminated the rear, and cracked the front bleeder like i mentioned a while ago, and the brakes loosened. Bingo.

I just had a thought. The current set up consists of a 1970’s Corvette style master cylinder. I’ve seen lots of cars and trucks use these when they upgrade to disc brakes. The MC piston has a place for the pedal pushrod for applications when a booster is not used. With a booster, a short piece of steel rod is inserted in the back of the MC piston to fill that hole and provide a place for the booster pushrod to push on to move the MC piston. If the steel rod is too long, then the master cylinder would never be able to fully retract to the home position but on the bench, the MC would work normally during the bench bleed. What do you think?
You can see the steel rod in question in this picture. Am I crazy?

Get a piece of cold rolled round stock the same diameter as the spacer, and cut off a piece about 1/8" shorter, and try a test drive with the short piece installed. You won't be making any sort of permanent modification that can't be reversed by simply reinstalling the original part.
Jerry

Ladies and gentlemen! I am happy to bring this story to an end.

The problem was the intermediate pushrod between the booster and the MC. It was approximately 1/8” too long. This was not noticeable when bolting the MC to the booster during assembly. However, once the MC was bolted to the booster, the piston in the MC was pushed in 1/8” and no longer at the “home” position. That means the MC piston was never able to properly release pressure from the front lines. So pressure in the front brakes would just keep building.

I removed the intermediate rod and functionally shortened it by drilling out the center about 1/8” so the nipple on the booster rod had more room. Put it all back together. Test drive and NO MORE DRAGGING OF THE BRAKES!

Time for a HAPPY DANCE!

Look at your photo of the booster- - - - -that looks a lot like a bolt and a lock nut where the plunger pushes the master cylinder. Dimpling the adapter accomplished the same thing, but in case you need to make any further adjustments, you can fine tune it in either direction without doing any more drilling.
Jerry

Yes, I tried to adjust that first while my wife patiently held the MC so it wouldn’t bend the brake lines. No matter what I did, it didn’t seem to change the booster rod tip length. So I resorted to drilling the spacer for the same result.

Don’t you just love it when a plan comes together. Good job.🛠

Congrats. Glad you found it.

Great diagnostic work! Congrats!!!