73 Duster front disc brakes dragging when hot

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Trevor B

Trevor B

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I've tried searching the forum for appropriate solutions but not having much luck so here goes:

1973 Duster with new master cylinder and booster, brake lines recently bled.
Front discs work fine when cold but start to drag when the brakes (get hot).
I can feel in the pedal that there is no play.

I took her out, got the brakes warm enough that I could feel it starting to drag, then quickly jacked up the front and felt that there was definitely resistance on both wheels. I could still turn each wheel by hand but it was not easy.

Ideas?

Thanks in advance for any tips/thoughts!
 
If not hoses, it could be the caliper sliders aren't sliding. They should be greased every pad change. Could also be "stuck calipers", which is common if the old fluid was rusty. Most people simply get rebuilt calipers since cheap, but if 73+ ones get expensive, you could rebuild them (new rubber seals & dust bellows).
 
Okay... the brake lines look really good. They were replaced in 2005 and the car sat for 9.5 years. I took off the calipers and greased the moving surfaces. The pads are basically new. I pulled the piston and inspected the sealing ring and dust boot - both look good.
Problem is still happening: when the brakes get hot they start to drag.

Here's something I forgot to mention that may be important: we put in a new master cylinder and brake booster. Could there be something in the linkage that could cause this?
 
The rod that comes out the front of the power brake booster needs to be adjusted.
Sounds like yours is a bit too long.
 
Was it originally a power brake car, or did you change it over?
C
 
Trevor B said:
Okay... the brake lines look really good. They were replaced in 2005 and the car sat for 9.5 years.
Click to expand...


Sat for nearly ten years. My guess is that moisture has built up and corroded the piston in the caliper, not allowing it to return.

AND, just because the hoses look good, does not mean they haven't collapsed on the inside, a common issue, something you ain't gonna see by looking at them.

Here is what I would try, make sure your bleeder screws are free, and you can loosen them. Take it out and warm them up as you did, jack it up, release the bleeder screw and see if the resistance goes away.

IF it does, the rubber lines are most likely the culprit.

If it does not, my money is still on the caliper seizing in the bore. I would completely flush the old fluid out, rebuild or replace both calipers add new fluid and bleed the system (I usually opt for replacement, they're fairly inexpensive and really important).

Conventional brake fluid attracts moisture, and really is a maintenance item, it should be flushed and refilled every couple of years. How many of us do it? Probably not a lot, but it should be done.
 
Would brake lines getting hot do this (headers) ?
 
It is either rubber lines, or the rebuilt master cylinder is junk
Had a problem on a Jeep CJ one time, the master cylinder was packaged wrong from NAPA.
You could only step on the brakes three times and it would lock the front brakes up.
The piston throw on the master cylinder was too long.
 
I don't think master cyl would be affected by hot brakes, wont it do it all the time. I with hoses and rebuild calibers.
 
I have 2 ideas for you, And they both depend on the brakes returning to normal by themselves, when they cool off.
-A) The fluid is not returning to the M/C. If the hoses ( you said both sides are doing this) are internally bad it prevents the fluid from returning through the compensating port, to relieve the pressure. A second possibility and more likely(its hard for me to accept that both hoses went at the same time) is that a) the pushrod between the pedal and the M/C is far enough out that the same thing is happening; the fluid is not returning, or b) the brake pedal itself is not parking at the top of its stroke, or c) the compensating port is plugged or severely restricted.
-B) the pads are too thick, or incorrectly installed such that even with the pistons fully retracted, theres just barely enough room to straddle the rotor.
-C) water in the caliper. When it heats up to boil, it flashes to steam and forces the pistons out against the pads, and into the rotor.This is usually not a problem until the caliper gets very hot. Usually requiring a lot of heat, in a short period of time.And would probably only manifest if the fluid was not returning as in A above.
--My money is on A-c, with maybe a touch of C
Time to get busy.Test the compensating port before doing anything else.
 
After much testing, head scratching (and banging), I have ordered new calipers and hoses.
My eyes and fingers tell me the pistons are in great shape. The calipers are cleaned and lubed. The sealing rings look and feel fine. Even the dust boots are serviceable.

When I pulled everything apart and cleaned it, the pistons moved smoothly although with resistance. Once fully compressed, there was plenty of room around the rotor (so the pads weren't too obviously too thick).

I adjusted the push rod between the booster and the MC, pulling it back slightly from the MC. Result: no power brakes. Adjusted it back toward the MC slightly and restored booster power, but the brakes stuck when I jammed my foot on the pedal hard a few times.

Thank you for all of your responses - so much wisdom on this forum.
I will post a follow up next week when my parts come and I can put them on.
 
Brakes work!
I took off the calipers again, cleaned them to within an inch of their lives, put on the $4 new sealing rings and dust boots, lubed them up, and put them back on. I should note that while the pistons are perfect, their socket in the calipers does have some slight pitting. But it's not near the rings and I smoothed everything out pretty good with steel wool.

Several adjustments of the booster's pushrod finally yielded something that works.

I drove the heck out of this car just now and those brakes are not hanging any more.

Thank you for all of your help. Now it's time to start adding horsepower!
 
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