brakes get tight after a few miles of driving?

The brakes on my dart, has non power disc brakes from a 73 dart, are getting tight and draging after about 4 miles of driving. The brake pedal has pretty much no travel when you push on it, it might move 1/4" seems like its pumped up hard all of the time. After the car sits a while they seem to work fine. I can jack the wheels up off the ground and they spin easy. I can push the brake pedal and release it and the wheels are still not tight. But if i do this after a few miles of driving i cant hardly spin the wheels with my hands, it takes a lot of muscle to make them spin and i will not even drift down my driveway after driving a few miles,
The calipers seem to be ok but it is like something is holding pressure on then when they warm up. Could the mastercylinder go bad and do this?

Sounds like your brake lines are heating up and the fluid is expanding. If you replaced the brake lines check the routing to make sure they aren't near the exhaust anywhere. Another thing I would verify is if it is all 4, just front, just rear or just a wheel, helps isolate which line(s) to look at. You also might check to see if the underhood heat is too high from something like headers or the engine getting too hot.

Good luck.

Sounds like the master cylinder is not returning all the way, Check the pushrod to piston clearance. should be .030 to .060

It happened to me. New master cylinder installed, truck drove well in the morning. By afternoon when the temperature was up, the brakes were locked. I had to bleed out some fluid to be able to drive home.

Adjusting the pushrod made it all OK.

R.

Were the front hoses new? Older hoses can swell on the inside and hold brake pressure on the calipers...

with a helper you might crack a bleeder to see if it is a psi prob or a piston/square seal (mechanical) caliper problem. If its psi then tighten the bleeder back up & being carefull not to intro air into the system & crack a line right at the MC next to the firewall (for the front discs) & see if that relaxes it which if so points to an internal MC prob as opposed to something downstream. Holler how it turns out. k.i.s.s. I'm thinking just bad calipers not retracting enough

X3 Adjustable pushrod?

Why would the push rod length matter with heat? It shouldn't change. Not arguing just trying to understand.

Your hoses are shot. Replace them.

From what I understand, as the fluid heats up, it has to expand somewhere. That somewhere is in the master cylinder, pushing back towards the pushrod. So having just a tiny bit of clearance there is enough to keep it from locking up.

I believe the piston can only move back to the stop/lock, correct? If the rod was pushing on the piston it would hold pressure cold or hot. I agree that fluid expands with heat. Just trying to wrap my head around it. Strangely enough my new to me RT was doing the same thing, I had to introduce some air into the system to be able to drive it home. I'll fix it when I get it here. I cant say that any of my other old cars experience anything like the pedal changing when they get warm. I built a Vette years ago that I had the issue with and it was an issue with the lines being too close, not to mention I have seen it a number of times professionally.

When the master cylinder piston moves back far enough, it uncovers a hole connected to the brake fluid reservoir. That allows the brake cylinders to relax backwards enough to cut most of the drag. Maybe it's easier to see in my single reservoir master cylinders.

If, when the piston moves back, that hole is still covered by the piston, fluid is trapped in the system. As the fluid heats up (in my case by the air temperature climbing), the fluid expands and applies pressure to the wheel cylinders, effectively locking the brakes. Cracking a line or bleeder allows some of the brake fluid to escape from the system, which allows the brake shoe springs to pull the shoes back and unlocks the brakes.

R.

What master cylinder are you using? I know I have modified some drum style for discs over the years... if it's a drum style, there's a valve that needs removed. Drum keep pressure and need the stop valve, while discs need to let the pressure off.

OK dogdays I can wrap my head around that, the rod has the piston in far enough to cover the hole, no consequence when the system is cold, even though it is holding slight pressure to the brakes. When it heats it would normally allow the fluid to push back into the master but since the piston is in the way it cant cause the pressure to apply the brakes. I've never looked at it that way but I could see it.

Silver70, my new RT is stock drums with a replacement but original style master. Since you are saying it essentially has a one way check valve would the rod still possibly be effecting my situation? The car is in PA and I'm in FL until Carlisle so I cant check it. I bought the car assembled so I cant say what the rod clearance was.

Thanks for the patience guys this has been good information. Much better when explained verse just it's the rod.

The compensating port in the master is staying blocked by the piston, cant return (because of rod length),thus not allowing the return of fluid to the reservoir It must have ample play between the rod an piston -----the fluid does expand with use.

I'm asking if you have a drum style master cylinder. It sounds as if disc brakes were swapped on the car. If you have a drum style master cylinder, it needs to be modified to release the pressure from the calipers. Drum and disc masters are not the same and the valve needs removed from a drum master to work with disc brakes. The valve is where the brake line goes into the master cylinder.

everything is the same on the car as it was when I restored it in 1993. the only thing I changed was the left front brake hose because the wheel was locking up and I determined that the hose was collapsing .I replaced the hose and now the caliper does not lock up. but both front brakes are getting tight after a few miles of driving, even if I don't use the brakes. the brakes were fine last summer but this year when I got the car out of the garage the brakes were not working right.
so I would think that since I have not changed anything since 1993 that I have the right parts but something just went bad. if I open the brake bleeder the brakes are loose again so I am thinking that the calipers are not rusted tight. I have just never heard of this problem before. but I guess after that many years I should just replace the hoses and master cylinder any way I guess.

What does it do if you break the line to the front brakes loose at the master? If it releases then the pressure is being held at the master. Breaking it loose at the caliper only eliminates the caliper.

if it ever stops raining here I will try driving a few miles then breaking it loose at the master and see what happens.

I finally figured out the brake issue. I replaced the master cylinder and drove over 100 miles with no problem.
I cant figure out what was wrong with the old master cylinder other than it has been on my car for 23 years and is old.

Did u adjust bp switch ?

I didn't adjust anything , just put on new mastercylinder and bleed the brakes.

Not every assembly is the same.
If your brake pushrod was pushed in the least amount
By the brake lite switch , it will spin freely when cold .
When hot , it will not disengage .
So .. The pedal may not ever have been completely released.