Brake drag. Need help.

To make a long story short, my front discs are dragging.

Long story is I have a 1973 scamp with Doctor diff front brake kit stage 1. New master cylinder and 1 piece proportioning valve from Doctor diff. At the track my car does not seem to be making the times that it should be. Certainly there are other factors but brake drag was brought up as a possible factor. Rebuilt rear drums. 512 ci RB motor. There is a line lock in place that appears to be functioning properly.

Other signs of brake drag are that after a 30 minute drive around town. Slowly. The front brake rotors are scalding hot. Sizzling if water touches them during a wash. I thought that it was the headers creating the heat,,, maybe not all of it. When the car is on jack stands, you can spin the tire with force but it stops immediately. The brake master cylinder had pressure that when I removed the cap to check levels, it popped . There was also fluid leaking out as though it was boiling. maybe not and I am imagining it but leaking nonetheless. The car does not coast very long in neutral. Finally, the rotors are super shiny, polished I would say.

Since everything is new, I did not know where to start. They rotors were tight from the beginning in that you could not turn the wheel freely from the get go. I was hoping that they would loosen up a bit.

My first checks to start today. Get the car up on stands.
1. open bleeders to check for residual pressure
2. put a dial gauge on the rotor to check for runout
3. loosen the caliper, press the brake pad all the way in, measure the opening and measure the rotor thickness. Maybe they are just too tight of tolerance and I need to turn the rotors down, say .005 in. each side.
4. While the calipers are off, I will check bearing tightness.
5. While the calipers are pressed in, put them back on without applying brake pressure and see if the wheels turn freely.
6. That should get me started, but I must be missing something. I will start this after church in a couple hours.

Please advise me if I am off track or missing something or any other information I can use. Fixing it will also need some advise and much of that depends on the diagnosis.

Thanks for the help in advance.

Bad front hoses or the master cylinder is not returning all the way.

Manual steering and manual drum brakes definitely the way to go with a bracket car. All disc brake cars will have some dragging - but yours is rather excessive.

Pushrod could be too long; same deal; compensating ports not open and fluid not returning.

To minimize drag,
I set my wheel-bearing adjustment to the max. This allows the discs to knock-back the pistons a couple of thou. This works on my fixed calipers, the KH 4-piston set-up. I would guess,not so much on the floaters. From 60mph, my car will coast down to 30ish in a half a mile or so, engine running, in Neutral, with a 4-speed,on concrete.

After you get it sorted, if it was me, I would deglaze the cooked pads. This is nothing more than shaving off the hardened part with emery-cloth.

Oldmanmopar said:

Bad front hoses or the master cylinder is not returning all the way.

Click to expand...

The hoses are new, How can you check if the master is not returning all the way out? Will it leave pressure in the lines such that when I open the bleeder it spurts out?

Is there anything else to check. I am going out in 15 minutes , breakfast time.

Kent mosby said:

Will it leave pressure in the lines such that when I open the bleeder it spurts out?

Click to expand...

yes

If your caliper pistons were pushed all the way back and the assembly was tight without brake fluid and bleeding from the get go, I would suspect a size / fit issue. If the tightness shows up once you've added brake fluid and bleeding I would be thinking a MC issue.

Well, that didn't help much. when I got the car on stands the wheels were not too bad. I could turn the rotors by hand after removing the wheels. We applied the brakes, released them and checked the bleeder valve. No residual pressure. Checked the line lock function and it released normally. Now the wheels coast at least one turn when giving them a shove. The calipers could be moved by small lever at least .025 inch. I did not use a feeler gauge but I was a machinist before going into dentistry.

Could an overfilled master cylinder cause the drag. Note that in my original post, there was pressure in the master cylinder when I removed the caps. So much pressure that the caps had leaked and overflowed spilling a bit. I cleaned them with brake cleaner yesterday and after yesterday’s drive there was a tiny tiny bit more. See the pic. I will take it for another drive and see if since I relieved the pressure in the master, the drag is gone. I will report back in 30 minutes. If they heat up and are tight after cruising around.

10 minute cruise we ran from 60 mph coast and it was about .4-.45 miles to get to 30. That does not sound like brake drag to me.. Thankfully, that tells me that the brakes, lines and such are good. After some more driving, I will check the drag again. It may still be a bad master cylinder. I will check for built up pressure in it whether or not there is any drag. This is just weird as the only thing I changed was to open the master cylinder caps and let out the pressure. The rotors were cool as well.

OR... The master cylinder is getting heated up by the TTI headers cooking the fluid... Would that cause brake drag? Anyone use a heat shield blanket like on a starter?

There shouldn't be any pressure or vacuum in the master cylinder reservoir. The caps should be vented to prevent that. Otherwise, as the brake linings wore and the fluid level dropped, it would create a slight vacuum, resulting in fluid not flowing into the cylinder chamber and air would get sucked into the system past any rubber seals downstream.

Actually caps are Not vented "exactly". The gasket serves as a bladder to "follow" the loss of fluid as the shoes / rotors wear. The metal cap is vented into the top of the gasket, but the gasket/ bladder completely seals the master. I've not experienced this, but I can see where it would be possible for heat in the engine bay to expand things enough that pressure would build in the master. Easy.......use a syringe/ etc to remove fluid. Drop it down 1/2" or so and try again. Normally, even if the master is "brimmed" the bulge in the tops of the cap should be enough "airspace"

I would be VERY suspicious of a long pushrod, or something "hanging up" so that master is not fully returning.

If return can not be fully confirmed, "this can get messy." Put a pan and LOTS of rags around / under the master, and leave the cap off or maybe lay it on "loose." Use a helper if you can. When you apply, then release brakes, you should see a "spurt" of fluid coming up out of the ports in the bottom of the master at each end. These are sometimes violent enough to spill over. "Go easy" with pedal release until you see what they will do

As said above there should be NO residual pressure in system with pedal relaxed.

You can easily check of the rotors don't have clearance. Remove one caliper at a time, and devise scrap wood whatever you need to retract piston. "Go easy." Make sure that when reinslalled there is plenty of pad clearance.

The pistons/ calipers do not have ANY method of retracting pads operationally, except for the tiny movement in the piston seals. They ?? deform to one side as pressure is applied, and are supposed to then return causing the pistons to "walk" in a tiny amount. Things like poor seal quality, poor condition of pistons/ calipers etc and various misalignment problems can diminish that little return

The side issue is that if you drive this enough with this probmem, and it might not take much of "enough" the shoes/ pads will glaze and stopping performance will suck

I checked the calipers/pads for clearance and we are good there. I will also take some of the fluid out as they were almost brimmed. The caps are not vented but there was nowhere for the pressure to go.
Thanks for telling me how to check for fluid return, I will check that soon. Hopefully, we are good as we are but I will check everything again.